Avoid leaking a shared worker global scope on a failed script load.

The spec tells us,

 http://whatwg.org/specs/web-apps/current-work/#run-a-worker

that in case of failure to load the script of a (shared) worker, we
should report the failure and abort (step 4.) The global scope's
closing flag will be left as false, meaning that we can safely shut
down and destroy the shared worker global scope. Do so.

R=
BUG=364390

Review URL: https://codereview.chromium.org/361443002

git-svn-id: svn://svn.chromium.org/blink/trunk@177178 bbb929c8-8fbe-4397-9dbb-9b2b20218538