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This was suggested in crbug.com/789636 so this logic can run in
production before the launch of the network service.

Had to keep the tab notification logic in each delegate, since the
way NetworkServiceNetworkDelegate maps the request back to the
original tab (using the URLLoader set as user data on the request)
does not work if network service is disabled.

Bug: 789636, 789632
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;luci.chromium.try:linux_mojo;master.tryserver.chromium.android:android_cronet_tester;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I22395f914639d8ce83457a63062927e876caeaa9
Reviewed-on: https://chromium-review.googlesource.com/1113903Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570877}