Fix leaks when the shared worker is requested to be terminated while loading the main script.

BUG=398742

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=180013

Review URL: https://codereview.chromium.org/451603002

git-svn-id: svn://svn.chromium.org/blink/trunk@180239 bbb929c8-8fbe-4397-9dbb-9b2b20218538