Worker: Early return if termination is requested while worker thread is starting

WorkerThread may get ready for shutdown if termination is requested while a
debugger task is running. In this case, WorkerThread disposes WorkerGlobalScope
etc, so it should not resume initialization.

BUG=615839

Review-Url: https://codereview.chromium.org/2075243002
Cr-Commit-Position: refs/heads/master@{#401543}
3 files changed