[Worker] Stop all workers in Blink shutdown sequence

Ensure all workers not to outlive the main thread.

BUG=374201,399803

Review URL: https://codereview.chromium.org/292173002

git-svn-id: svn://svn.chromium.org/blink/trunk@181309 bbb929c8-8fbe-4397-9dbb-9b2b20218538