Follow-up fixes for WorkerThread.

In my initial patch (https://codereview.chromium.org/438523002/) to move WorkerThread to WebThread there were two issues:
* When WorkerThread::stop is called, it doesn't kill the debug message queue
* The shared timer didn't account for WorkerGlobalScope in closing state.

This CL fixes those two issues.

BUG=301515

Review URL: https://codereview.chromium.org/436663002

git-svn-id: svn://svn.chromium.org/blink/trunk@179339 bbb929c8-8fbe-4397-9dbb-9b2b20218538