Worker: Support ES Modules on DedicatedWorker behind the runtime flag

This CL supports ES Modules on DedicatedWorker behind the ModuleDedicatedWorker
flag and adds WPT tests.

With the flag, you can specify 'type' option on the constructor of
DedicatedWorker (i.e., new Worker) to start a dedicated worker as a module
script. On DedicatedWorkerGlobalScope, static import is available, but dynamic
import() hasn't been implemented yet.

Bug: 680046
Change-Id: I3aca350228ec07be7884c7a2eb8cd351e7fd6b6e
Reviewed-on: https://chromium-review.googlesource.com/967908
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544983}
9 files changed