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This should get rid of the spurious ERROR messages when closing tabs, and log backtraces when we need them.

BUG=141055


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=151975

Review URL: https://chromiumcodereview.appspot.com/10834355

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@152395 0039d316-1c4b-4281-b951-d872f2087c98