Move NetworkConnectionTracker getter to network_service_instance.h

The NetworkConnectionTracker instance currently lives in
g_browser_process, but we need the tracker in //content, which can't
access //chrome. This adds a static getter in
network_service_instance.h, which can be used in //content.

Bug: 821009
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: Iaedb89cf2d27f75e0b896d7fd7389ceab3fe4682
Reviewed-on: https://chromium-review.googlesource.com/1162888
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Robbie McElrath <rmcelrath@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581697}
24 files changed