Worker: Create WebWorkerFetchContext in DedicatedWorker::CreateGlobalScopeCreationParams()

This CL moves WebWorkerFetchContext creation from
ThreadedMessagingProxy::StartGlobalScope() to
Dedicatedworker::CreateGlobalScopeCreationParams() in order to put params
construction in one place for code cleanup.

Bug: 906991
Change-Id: I7a365bbc0706f271b61e9f56648f9b7a47506dd8
Reviewed-on: https://chromium-review.googlesource.com/c/1351199
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611553}
3 files changed