GitLab

This is so that callers can code against a TaskRunner interface
directly, rather than converting message_loop_proxy() into a TaskRunner.

BUG=391045

Review URL: https://codereview.chromium.org/389653005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282767 0039d316-1c4b-4281-b951-d872f2087c98