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This change replaces the NewTabPageInterceptorService with a
NewTabPageNavigationThrottle.  The throttle reduces complexity, but it
is also neutral regarding the network stack / network service, so it
fixes the NewTabPageInterceptorServiceTests that were broken with the
NetworkService feature enabled.  These tests are now renamed
NewTabPageNavigationThrottleTest.

BUG=802926

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: Iec0664df33e79d6bcc18f60aecc4017de7f65858
Reviewed-on: https://chromium-review.googlesource.com/868994
Commit-Queue: Conley Owens <cco3@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#530370}