Have the Worker destructor handle non-started Workers better.

If the creation of a Worker fails early due to an invalid script URL, the
worker context proxy isn't created. Hence, the assumption that the
Worker's ExecutionContext is kept alive by it doesn't apply. Adjust
the asserting in the destructor to reflect that.

R=haraken@chromium.org
BUG=301575

Review URL: https://codereview.chromium.org/370553003

git-svn-id: svn://svn.chromium.org/blink/trunk@177474 bbb929c8-8fbe-4397-9dbb-9b2b20218538