(Retry) Worker: Implement WorkerGlobalScope::ImportModuleScript()

The original CL was reverted by a following CL because of compile failures on
bots:
https://chromium-review.googlesource.com/c/chromium/src/+/836187

This CL fixes the failures by rebase. Also, this moves
ParseCredentialsMode() from anonymous namespace to static member of
DedicatedWorkerMessagingProxy to fix jumbo build failures because of function
name collision with ParseCredentialsMode() in Worklet.cpp.

===== Original CL description =====

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: I2ec26bdeb526fa5925bb37adfa07fde0590b2a41
TBR: kouhei@chromium.org,kinuko@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/836228
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#525619}
9 files changed