Verifies that mixed content issue is created from mixed content iframe.

Inspector issue: {
    issue : {
        code : MixedContentIssue
        details : {
            mixedContentIssueDetails : {
                frame : {
                    frameId : <string>
                }
                insecureURL : http://devtools.test:8000/inspector-protocol/resources/iframe.html
                mainResourceURL : https://devtools.test:8443/inspector-protocol/resources/mixed-content-iframe.html
                resolutionStatus : MixedContentBlocked
                resourceType : Frame
            }
        }
    }
}