Reland "Fix flakiness in previews browsertests"

This reverts commit de34f0bdf289e3b788f7a09be8ae97c45d1e9832.

Reason for revert: Fixed additional flakiness

Original change's description:
> Revert "Fix flakiness in previews browsertests"
>
> This reverts commit ddd4f721f82a2b4065d214eee99e9fe28199ee15.
>
> Reason for revert: Looks like some tests become flaky (or even flakier?)
>
> https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/linux-chromeos-dbg/10417
> https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/linux-chromeos-dbg/10418
> https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/linux-chromeos-dbg/10419
> https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Linux%20Chromium%20OS%20ASan%20LSan%20Tests%20%281%29/31176
>
> Original change's description:
> > Fix flakiness in previews browsertests
> >
> > The previous logic relied upon the hints requested by the store upon a
> > navigation to a url being loaded prior to the commit. This was not
> > guaranteed to happen when cores were not available on a machine for
> > background threads.
> >
> > The HintCache has been modified so that it now runs the hint loaded
> > callback even in the case where no hints are available for the host.
> >
> > Additionally, a histogram has been added that tracks the loading of
> > hints from the hint cache store has in PreviewsOptimizationGuide. The
> > tests now run a priming navigation that waits for the hints to be
> > loaded. This ensures that the hints are guaranteed to be available
> > during the tests.
> >
> > Bug: 923161
> > Change-Id: I8b09dfc9391164f0a6d424ec0a8ea871334c4c8c
> > Reviewed-on: https://chromium-review.googlesource.com/c/1423617
> > Commit-Queue: Jered Gray <jegray@chromium.org>
> > Reviewed-by: Tarun Bansal <tbansal@chromium.org>
> > Reviewed-by: Doug Arnett <dougarnett@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#625056}

Change-Id: I675f7a0b3eb79d468100af8775310bb64531b069
Reviewed-on: https://chromium-review.googlesource.com/c/1431019
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Doug Arnett <dougarnett@chromium.org>
Commit-Queue: Jered Gray <jegray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625352}
7 files changed