The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1525dc5d06e2b4a1ce3d53fdc8f4c01c46174fa3

commit 1525dc5d06e2b4a1ce3d53fdc8f4c01c46174fa3
Author: John Abd-El-Malek <jam@chromium.org>
Date: Tue Sep 04 19:24:53 2018

Fix content_browsertests on Mac with network service.

The problem was NetworkServiceTestHelper wasn't instantiated. Unlike Mac/Linux where content_browsertests child processes are launched in content/test/content_test_launcher.cc (which tells ShellMainDelegate that it's running in test mode), Mac uses content/shell/app/shell_content_main.cc.

Bug:  877996 
Change-Id: I7c6ca51d9080064e06a7d333bcc725a8de9d2d25
Reviewed-on: https://chromium-review.googlesource.com/1204273
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588610}
[modify] https://crrev.com/1525dc5d06e2b4a1ce3d53fdc8f4c01c46174fa3/content/shell/app/shell_content_main.cc
[modify] https://crrev.com/1525dc5d06e2b4a1ce3d53fdc8f4c01c46174fa3/content/shell/browser/shell_content_browser_client.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/69ae1491f407b33f1ee696095a1cbe12f18d22ca

commit 69ae1491f407b33f1ee696095a1cbe12f18d22ca
Author: John Abd-El-Malek <jam@chromium.org>
Date: Thu Sep 06 20:00:06 2018

Run network service browser tests on Mac 10.12 bots.

This only runs on the main waterfall, as the CQ is on 10.13. This is fine for now, given that network service is on the CQ for Windows and Linux, and there's no Mac specific networking code that this impacts.

Bug:  877996 
Change-Id: I955c397920f307869c2b1c5b84a62ae620b6be9d
Reviewed-on: https://chromium-review.googlesource.com/1200308
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589256}
[modify] https://crrev.com/69ae1491f407b33f1ee696095a1cbe12f18d22ca/chrome/browser/media/webrtc/webrtc_browsertest.cc
[modify] https://crrev.com/69ae1491f407b33f1ee696095a1cbe12f18d22ca/testing/buildbot/chromium.clang.json
[modify] https://crrev.com/69ae1491f407b33f1ee696095a1cbe12f18d22ca/testing/buildbot/chromium.mac.json
[modify] https://crrev.com/69ae1491f407b33f1ee696095a1cbe12f18d22ca/testing/buildbot/test_suite_exceptions.pyl
[modify] https://crrev.com/69ae1491f407b33f1ee696095a1cbe12f18d22ca/testing/buildbot/test_suites.pyl