Tests for browser.cancelDownload
## Expected usage
Page.downloadWillBegin: {
    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&tail_wait=10000&size=4096
    }
    sessionId : <string>
}
Page.downloadProgress (change of state): {
    method : Page.downloadProgress
    params : {
        guid : <string>
        receivedBytes : <number>
        state : inProgress
        totalBytes : 0
    }
    sessionId : <string>
}
Page.downloadProgress (change of state): {
    method : Page.downloadProgress
    params : {
        guid : <string>
        receivedBytes : <number>
        state : canceled
        totalBytes : 0
    }
    sessionId : <string>
}
Browser.cancelDownload: {
    id : <number>
    result : {
    }
    sessionId : <string>
}
## Trying already terminated downloads
Browser.cancelDownload: {
    id : <number>
    result : {
    }
    sessionId : <string>
}
## Trying invalid GUID
Browser.cancelDownload: {
    error : {
        code : -32602
        message : No download item found for the given GUID
    }
    id : <number>
    sessionId : <string>
}