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This creates a mostly default NetworkContext. This duplicates the work
provided by the content-layer implementation, but will later be extended
to provide more functionality.

The main addition beyond content layer is this configures WebView's
HTTP cache.

R=changwan@chromium.org, mmenke@chromium.org

Cq-Include-Trybots: master.tryserver.chromium.android:android_mojo
Bug: 887538
Test: No major issues with NetworkService feature enabled.
Change-Id: I9cc1823f144d9ffc96fca917b0763b74e36a53d7
Reviewed-on: https://chromium-review.googlesource.com/c/1314133
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Changwan Ryu <changwan@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605508}