Worker: Implement WorkerGlobalScope::ImportModuleScript()

This CL implements WorkerGlobalScope::ImportModuleScript() that imports module
scripts using WorkerModulatorImpl, and wires up DedicatedWorkerMessagingProxy
via WorkerThread.

Note that module workers are still not available because WorkerOptions is not
exposed on the ctor of Worker and WorkerModulatorImpl is not fully implemented
yet.

Design doc: https://docs.google.com/a/chromium.org/document/d/1IMGWAK7Wq37mLehwkbysNRBBnhQBo3z2MbYyMkViEnY/edit?usp=sharing

Bug: 680046
Change-Id: I0c616b7a1b79abb02c1085be356293510dd4c04b
Reviewed-on: https://chromium-review.googlesource.com/807675
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#525270}
8 files changed