Fix SharedWorker leak when script load has failed.

When SharedWorker script load has failed and the |WebSharedWorkerImpl|
was explicit told to terminate, the |WebSharedWorkerImpl| instance
was never deleted.

This CL fixes the issue by invoking "delete this" when a worker thread
instance does not exist in |stopWorkerThread()|.

TEST=fast/workers/worker-crash-with-invalid-location.html doesn't leak when --enable-leak-detection
BUG=364390

Review URL: https://codereview.chromium.org/424653002

git-svn-id: svn://svn.chromium.org/blink/trunk@179011 bbb929c8-8fbe-4397-9dbb-9b2b20218538