Worker: Implement WorkerModuleFetchCoordinator

This CL implements WorkerModuleFetchCoordinator that actually initiates module
loading on the main thread in response to requests from
WorkerOrWorkletModuleFetcher on the worker thread.

Design doc:
https://docs.google.com/document/d/1IMGWAK7Wq37mLehwkbysNRBBnhQBo3z2MbYyMkViEnY/edit#heading=h.637avx8i5qtn

Bug: 680046
Change-Id: Ib845662436356b2966ed987a88be33ed5dc970f8
Reviewed-on: https://chromium-review.googlesource.com/963874
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544002}
13 files changed