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When running network service without launching full browser process,
ChromeContentBrowserClient::OnNetworkServiceCreated() will be called
without g_browser_process. This CL allows us to create
SystemNetworkContextManager on demand and services depending on the
network service can call SystemNetworkContextManager::GetInstance()
to access it.

BUG=866028

Change-Id: I9a557921e1bb164ecd62c766dd698137498c0a47
Reviewed-on: https://chromium-review.googlesource.com/1256207
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596269}