Adds support for chrome-extension:// URLs when the Network Service
is enabled. Navigation and subresource loads are supported, with
access policy mirroring the non-network service path.

Unlike the old code, this loading path does not run on the IO thread
and has no dependencies on IO thread data.

To support this change while reusing significant amounts of core logic,
some extensions code has been refactored to be thread agnostic with
more granular data dependencies (e.g. taking a GURL and ExtensionSet
instead of net::URLRequest and extensions::InfoMap*).

The net result of this change is that we can re-enable ~1220
browser_tests with Network Service enabled.

Bug: 780325
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_site_isolation
Change-Id: I7143c1bf080f40a6f00fc761ed823add951dd61e
Reviewed-on: https://chromium-review.googlesource.com/756660


Commit-Queue: Ken Rockot <rockot@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#514915}