Worker: Directly create subresource loader factory bundle from bundle info passed by the browser

This CL makes EmbeddedSharedWorkerStub directly create
ChildURLLoaderFactoryBundle for subresource loading from the bundle info passed
by the browser process. This is aligned with what
DedicatedWorkerHostFactoryClient does.

Before this CL, the stub creates HostChildURLLoaderFactoryBundle first, and then
updates it with the given bundle info.

Bug: n/a
Change-Id: I943654bed63ad7abf7411471ccdb63cd0b94c10a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1531895
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643683}