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This allows all CdmPromise objects to report their result (success or
exception) if given the name of a histogram at construction.

As an example, a UMA for the unprefixed NewSession method is included.

BUG=351501

Review URL: https://codereview.chromium.org/452643002

Cr-Commit-Position: refs/heads/master@{#289453}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289453 0039d316-1c4b-4281-b951-d872f2087c98