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This adds a task graph interface to the worker pool and
implements a simple queue instance of this interface for
use by the tile manager.

The task graph interface can be used describe more
complicated task dependencies in the future and
provides the immediate benefit of seamlessly being
able to cancel and re-prioritize tasks.

BUG=178974

Review URL: https://chromiumcodereview.appspot.com/14689004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@202363 0039d316-1c4b-4281-b951-d872f2087c98