Revert of Signal WorkerThread termination earlier (patchset #1 id:1 of https://codereview.chromium.org/667813003/)
Revert of Signal WorkerThread termination earlier (patchset #1 id:1 of https://codereview.chromium.org/667813003/)

Reason for revert:
This seems to cause a crash: http://crbug.com/426063

Original issue's description:
> Signal WorkerThread termination earlier
> 
> This CL resolves a race in WorkerThread::cleanup.
> workerThreadTerminated() notification may release a reference to the WorkerThread instance, that causes race between the termination signal.
> 
> BUG=405420
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=184061

TBR=haraken@chromium.org,aandrey@chromium.org,sigbjornf@opera.com
NOTREECHECKS=true
NOTRY=true
BUG=405420,426063

Review URL: https://codereview.chromium.org/671913003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@184206 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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