Implement ResourceQueue, an object that makes it easy to delay starting

      requests in ResourceDispatcherHost until specified conditions are met.

Make UserScriptListener use ResourceQueue.

This is the first step toward waiting for the privacy blacklists to load.

TEST=Covered by unit_tests.
BUG=21541

Review URL: http://codereview.chromium.org/460108

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@34271 0039d316-1c4b-4281-b951-d872f2087c98