SharedWorker: Introduce "constructor origin" concept defined in the spec

This CL introduces the "constructor origin" concept defined in the HTML spec:
https://html.spec.whatwg.org/multipage/workers.html#concept-sharedworkerglobalscope-constructor-origin

Bug: 787103
Change-Id: I273629760bb34e0c24f1c4d023e66e146a476407
Reviewed-on: https://chromium-review.googlesource.com/781539
Reviewed-by: Raymes Khoury <raymes@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520417}