Rename workerGlobalScopeDestroyed to workerThreadTerminated

This is a follow-up fix for https://codereview.chromium.org/462423003/.
It's inconsistent that workerGlobalScopeDestroyed is called while
workerGlobalScopeStarted is not called. In fact, workerGlobalScopeDestroyed
is a signal to let the WorkerMessagingProxy know that the worker thread
is terminated, it should be renamed to workerThreadTerminated.

BUG=403179

Review URL: https://codereview.chromium.org/468883002

git-svn-id: svn://svn.chromium.org/blink/trunk@180254 bbb929c8-8fbe-4397-9dbb-9b2b20218538