Revert 179604 "WorkerThread's pointer should be protected until ..."

I committed another change at the same time by mistake.

> WorkerThread's pointer should be protected until all the shutdown tasks are complete
> 
> During destruction of WorkerThread, m_pendingGCRunner is destroyed prior to m_thread,
> because it is following it in the members ordering in the class.
> So when m_thread is being destroyed, it is waiting for all the tasks on the message loop
> to complete, some of which need the PendingGCRunner. This will cause use-after-free.
> 
> To prevent the situation, this CL adds code to keep the WorkerThread pointer alive
> until we complete all the shutdown tasks.
> 
> BUG=400511
> TEST=none (I cannot reproduce the crash)
> R=ager@chromium.org, jochen@chromium.org, nasko@chromium.org, zerny@chromium.org
> 
> Review URL: https://codereview.chromium.org/445863002

TBR=haraken@chromium.org

Review URL: https://codereview.chromium.org/449433002

git-svn-id: svn://svn.chromium.org/blink/trunk@179608 bbb929c8-8fbe-4397-9dbb-9b2b20218538