Fix UAF crash bug in WebEmbeddedWorkerImpl.

This crash occurred when WebEmbeddedWorkerImpl::terminateWorkerContext() is called while loading the main script.
See: http://crbug.com/400012#c4

BUG=400012

Review URL: https://codereview.chromium.org/449533003

git-svn-id: svn://svn.chromium.org/blink/trunk@179771 bbb929c8-8fbe-4397-9dbb-9b2b20218538