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

This reverts commit 2864f69f22b4c9398a49f059b72b28f9bece8fc3.

Reason for revert: http://crbug.com/914724

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}

TBR=jam@chromium.org,mmenke@chromium.org

Change-Id: I6506e560dea13058d0336e22c3fc4844ac57fa04
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 837753, 914724
Reviewed-on: https://chromium-review.googlesource.com/c/1376101
Reviewed-by: Clark DuVall <cduvall@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#616349}(cherry picked from commit ebc1d4a86e895b51e87f163b2b34a98a7325e866)
Reviewed-on: https://chromium-review.googlesource.com/c/1376102
Cr-Commit-Position: refs/branch-heads/3639@{#4}
Cr-Branched-From: 4649b0ba5c9760073f53a859e782555c4c0e28e8-refs/heads/master@{#616124}
11 files changed