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Breaks chrome_frame_tests on Win Aura.
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Chromium&testType=chrome_frame_tests&builder=Win%20Aura%20Tests%20(1)

> Add UMA to report Preferences File Corruption
> Adds PrefMetricsService methods to track changes to user preferences
> that happen outside of Chrome and report discrepancies via UMA.
> 
> BUG=266569
> 
> Review URL: https://chromiumcodereview.appspot.com/22676002

TBR=bbudge@chromium.org

Review URL: https://codereview.chromium.org/23597020

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@221966 0039d316-1c4b-4281-b951-d872f2087c98