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Previously, they were initialized on first use, however, now that
initialization sets up NetworkContexts when the network service is
disabled, relying on the first use of the class through the
ChromeURLRequestContextGetter API before setting up the NetworkContext
could cause problems.

Bug: 847555
Change-Id: I2aaac7e717e64fb41ba2fb9141506bb7512b73fa
Reviewed-on: https://chromium-review.googlesource.com/1169319Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582188}