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The MessageLoop has some code for adding debug annotations such as trace
flows and memory usage tags to posted tasks. Since we want to use these
annotations more generally for tasks that are scheduled outside the base
message loop, this patch factors it out into a standalone class.

TEST=TaskAnnotatorTest
BUG=391005

Review URL: https://codereview.chromium.org/455833004

Cr-Commit-Position: refs/heads/master@{#289560}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289560 0039d316-1c4b-4281-b951-d872f2087c98