Have WorkerScriptController::workerGlobalScope return a reference

Have WorkerScriptController::workerGlobalScope return a reference as it
can never return null. It makes the calling code look safer.

BUG=

Review URL: https://codereview.chromium.org/27552003

git-svn-id: svn://svn.chromium.org/blink/trunk@159970 bbb929c8-8fbe-4397-9dbb-9b2b20218538