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This adds the basic triggering logic to the Previews Navigation Throttle
(not including loadshedding or bypassing which will come in the next
CL).
Also creates a NavigationThrottleManager interface to decouple the Nav
Throttle from the decider. The decider will keep all the state for the
Nav Throttle since the throttle's lifetime is so short.

Most of the triggering testing is done in browser tests for sake of
clarity and complexity.

Bug: 864651
Change-Id: I1d1772c25ddfa159051bd5b1064c53256ced39f3
Reviewed-on: https://chromium-review.googlesource.com/1152164
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Doug Arnett <dougarnett@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582220}