Worker: Deduplicate WebWorkerFetchContext creation code

There are duplicate code between RenderFrameImpl::CreateWorkerFetchContext() and
EmbeddedSharedWorkerStub::CreateWorkerFetchContext(). This CL factors them out
into WebWorkerFetchContext::Create() for cleanup.

Bug: 906991
Change-Id: Ic912e6b7b62777c2334ec51e21036b129296eb1a
Reviewed-on: https://chromium-review.googlesource.com/c/1405123
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622802}
4 files changed