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This CL adds a utility class to streamline the task of testing that metrics have been updated as expected. Specifically, the HistogramRecorder class allows a client to obtain the differential value of a given histogram in the interval since the given HistogramRecorder instance was created.

BUG=232414

Review URL: https://chromiumcodereview.appspot.com/18337014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@212378 0039d316-1c4b-4281-b951-d872f2087c98