Tests we properly emit Page.downloadWillBegin.
Downloading via a navigation: 
downloadWillBegin from top frame: {
    method : Page.downloadWillBegin
    params : {
        frameId : <string>
        guid : <string>
        suggestedFilename : hello.txt
        url : http://127.0.0.1:8000/devtools/network/resources/resource.php?download=1
    }
    sessionId : <string>
}
{
    method : Page.downloadProgress
    params : {
        guid : <string>
        receivedBytes : <number>
        state : inProgress
        totalBytes : 11
    }
    sessionId : <string>
}
{
    method : Page.downloadProgress
    params : {
        guid : <string>
        receivedBytes : <number>
        state : completed
        totalBytes : 11
    }
    sessionId : <string>
}
Downloading via a navigation of subframe: 
downloadWillBegin from child frame: {
    method : Page.downloadWillBegin
    params : {
        frameId : <string>
        guid : <string>
        suggestedFilename : hello.txt
        url : http://127.0.0.1:8000/devtools/network/resources/resource.php?download=1
    }
    sessionId : <string>
}
{
    method : Page.downloadProgress
    params : {
        guid : <string>
        receivedBytes : <number>
        state : inProgress
        totalBytes : 11
    }
    sessionId : <string>
}
{
    method : Page.downloadProgress
    params : {
        guid : <string>
        receivedBytes : <number>
        state : completed
        totalBytes : 11
    }
    sessionId : <string>
}
Downloading by clicking a link: 
downloadWillBegin from top frame: {
    method : Page.downloadWillBegin
    params : {
        frameId : <string>
        guid : <string>
        suggestedFilename : hello.text
        url : http://127.0.0.1:8000/devtools/network/resources/resource.php
    }
    sessionId : <string>
}
{
    method : Page.downloadProgress
    params : {
        guid : <string>
        receivedBytes : <number>
        state : inProgress
        totalBytes : 11
    }
    sessionId : <string>
}
{
    method : Page.downloadProgress
    params : {
        guid : <string>
        receivedBytes : <number>
        state : completed
        totalBytes : 11
    }
    sessionId : <string>
}
Downloading by clicking a link in subframe: 
downloadWillBegin from child frame: {
    method : Page.downloadWillBegin
    params : {
        frameId : <string>
        guid : <string>
        suggestedFilename : hello.text
        url : http://127.0.0.1:8000/devtools/network/resources/resource.php
    }
    sessionId : <string>
}
{
    method : Page.downloadProgress
    params : {
        guid : <string>
        receivedBytes : <number>
        state : inProgress
        totalBytes : 11
    }
    sessionId : <string>
}
{
    method : Page.downloadProgress
    params : {
        guid : <string>
        receivedBytes : <number>
        state : completed
        totalBytes : 11
    }
    sessionId : <string>
}
Downloading by clicking a link (no HTTP Content-Disposition header, a[download=""]): 
downloadWillBegin from top frame: {
    method : Page.downloadWillBegin
    params : {
        frameId : <string>
        guid : <string>
        suggestedFilename : download.zzz
        url : http://127.0.0.1:8000/devtools/network/resources/download.zzz
    }
    sessionId : <string>
}
Downloading by clicking a link (HTTP Content-Disposition header with filename=foo.txt, no a[download]): 
downloadWillBegin from top frame: {
    method : Page.downloadWillBegin
    params : {
        frameId : <string>
        guid : <string>
        suggestedFilename : foo.txt
        url : http://127.0.0.1:8000/devtools/network/resources/resource.php?named_download=foo.txt
    }
    sessionId : <string>
}
Downloading by clicking a link (HTTP Content-Disposition header with filename=override.txt, a[download="foo.txt"]): 
downloadWillBegin from top frame: {
    method : Page.downloadWillBegin
    params : {
        frameId : <string>
        guid : <string>
        suggestedFilename : override.txt
        url : http://127.0.0.1:8000/devtools/network/resources/resource.php?named_download=override.txt
    }
    sessionId : <string>
}
Downloading by clicking a link (HTTP Content-Disposition header without filename, no a[download]): 
downloadWillBegin from top frame: {
    method : Page.downloadWillBegin
    params : {
        frameId : <string>
        guid : <string>
        suggestedFilename : resource.txt
        url : http://127.0.0.1:8000/devtools/network/resources/resource.php?named_download
    }
    sessionId : <string>
}
Downloading by clicking a link (HTTP Content-Disposition header without filename, no a[download], js): 
downloadWillBegin from top frame: {
    method : Page.downloadWillBegin
    params : {
        frameId : <string>
        guid : <string>
        suggestedFilename : resource.js
        url : http://127.0.0.1:8000/devtools/network/resources/resource.php?named_download&type=js
    }
    sessionId : <string>
}
Downloading by clicking a link (HTTP Content-Disposition header without filename, no a[download], image): 
downloadWillBegin from top frame: {
    method : Page.downloadWillBegin
    params : {
        frameId : <string>
        guid : <string>
        suggestedFilename : resource.png
        url : http://127.0.0.1:8000/devtools/network/resources/resource.php?named_download&type=image
    }
    sessionId : <string>
}