Extensions: Hide network requests from the NTP renderer.

The NTP (New tab page) renderer is a semi-priveleged process having access to
some special APIs. But currently, network requests made by an NTP renderer can
be intercepted by extensions using the web request API.

This CL hides network requests made by an NTP renderer from extensions. This
works since it's guaranteed that the NTP won't share the renderer process with
any other site.

BUG=797461

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: Id8523aa7cbb4e5fb74256fa6cfc180709849568a
Reviewed-on: https://chromium-review.googlesource.com/1018724
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553265}
18 files changed