Worker: Clean up WorkerClients handling

Before this CL, WorkerClients is indirectly passed to GlobalScopeCreationParams
via MessagingProxy. For example, DedicatedWorker creates WorkerClients
and passes it to GlobalScopeCreationParams via DedicatedWorkerMessagingProxy.

This indirection is no longer necessary thanks to recent cleanups. This CL
makes DedicatedWorker etc directly pass WorkerClients to
GlobalScopeCreationParams and removes the indirection paths.

Bug: 680046
Change-Id: Ie20885f45961cb83503f041b68a2229f4c832f10
Reviewed-on: https://chromium-review.googlesource.com/832603
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526269}
16 files changed