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This patch adds notification methods for start/stop of worker threads which accept a WorkerThread parameter. This is needed in order to move the worker threads in blink to use WebThread instead of its own version.
It also adds support for shared timer to WebThreadImpl.

BUG=301515

Review URL: https://codereview.chromium.org/393283003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284008 0039d316-1c4b-4281-b951-d872f2087c98