WorkerThread should not start execution if WorkerThread::stop is already called

If WorkerThread::stop is already called, WorkerThread::initialize should return immediately without starting the thread execution.

BUG=403179
TEST=none (it's timing-dependent)

Review URL: https://codereview.chromium.org/462423003

git-svn-id: svn://svn.chromium.org/blink/trunk@180139 bbb929c8-8fbe-4397-9dbb-9b2b20218538