SafeBrowsingBlockingPageTest.MalwareIframeReportDetails: use MessageLoopRunner for waiting for report to be sent.

BUG=273302

Review URL: https://codereview.chromium.org/180013007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256784 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256784