GitLab

What's removed:
 - The idea of unique_id's (these were for the now-gone webUI taskmanager).
 - The GetResourceExtension() method, which the tests no longer need.
 - The IsBackground getter, which is not used.

BUG=348836

Review URL: https://codereview.chromium.org/186153002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255217 0039d316-1c4b-4281-b951-d872f2087c98