Use TargetIsServiceWorker in ServiceWorker script loading

As now chromium side's patch is landed:
https://src.chromium.org/viewvc/chrome?revision=243736&view=revision

BUG=313530

Review URL: https://codereview.chromium.org/130913003

git-svn-id: svn://svn.chromium.org/blink/trunk@164859 bbb929c8-8fbe-4397-9dbb-9b2b20218538