Handle a terminating context in messageHandlerinWorker().

If execution is terminated while constructing the ErrorEvent representing
the incoming v8::Message, do not attempt to report the exception.

R=haraken@chromium.org
BUG=393273

Review URL: https://codereview.chromium.org/405933002

git-svn-id: svn://svn.chromium.org/blink/trunk@178929 bbb929c8-8fbe-4397-9dbb-9b2b20218538