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The end goal is to get NavigationSimulator to a state where we can call:
simulator->SetAutoAdvance(false);
simulator->Start();

And have the simulator *not* spin the run loop in the background. This
will help in writing more sophisticated NavigationThrottle tests which
use multiple task runners to execute async tasks.

This particular CL does a few things:
1. Removes base::RunLoop().RunUntilIdle call. This is not great practice
   to have in test infra, since it can cause strange interactions with
   tests and they may start relying on unintentional behavior.

2. Collects various after-throttle checks into closures, to be called
   when the throttle check complete closure is called (rather than
   relying on base::RunLoop to keep everything on the stack). This
   isn't so useful now, but it will if we stop *always* waiting.

3. Slightly modifies checks in various places that relied on some of
   the behavior that we've changed. Two things changed here:
   a. "Complete" code internal to the NavigationThrottle now runs
      directly after throttle checks are complete (when
      complete_callback_for_testing_ is called on the nav handle),
      rather than when the RunLoop quits. This caused us to remove
      one check checking that we've finished the navigation.

   b. Because we no longer RunUntilIdle() in
      WaitForThrottleChecksComplete, code that relies on the nav finishing
      after the post-task in NavigationRequest::OnStartChecksComplete
      will need to either wait or spin the run loop to see the end of the
      navigation. This is a pretty small edge case and it is pretty
      easy to either write a helper to make this explicit or use a WCO.
      The only test impacted by this change is a NavigationSimulator test,
      and a test which incidentally relied on this behavior for unrelated
      reasons.

Bug: 822275

Change-Id: Ice6b215fa285a6a944fdf85108303515d109be77
Reviewed-on: https://chromium-review.googlesource.com/961819Reviewed-by: oysteine <oysteine@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546184}