Reland "Stop creating net::URLRequestContext in the browser process when network service is enabled."

This is a reland of 2864f69f22b4c9398a49f059b72b28f9bece8fc3

Patchset 1->2 has the fix for not attempting to create the URLRequestContext in the browser in Clank.

Original change's description:
> Stop creating net::URLRequestContext in the browser process when network service is enabled.
>
> Bug: 837753
> Change-Id: I479666949f8c552e964ab86420ef080db9010d06
> Reviewed-on: https://chromium-review.googlesource.com/c/1367088
> Commit-Queue: John Abd-El-Malek <jam@chromium.org>
> Reviewed-by: Matt Menke <mmenke@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#615995}

Bug: 837753
Change-Id: Id9429da79b53cc3dd2292affdb411cf1c055b1f3
Reviewed-on: https://chromium-review.googlesource.com/c/1376319
Reviewed-by: Clark DuVall <cduvall@chromium.org>
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616824}
15 files changed