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The loop action plays HTML5 media in loops and measures the avg time it 
takes to loop, i.e. time from end to start playback again.

BUG=249435

Review URL: https://chromiumcodereview.appspot.com/21134002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@220682 0039d316-1c4b-4281-b951-d872f2087c98