Deflake resource loading hints and previews browsertest

Attempt to deflake resource loading hints and previews
browsertest by waiting for hints to be processed.

Currently, the hints are processed on the background thread,
and so may not be processed immediately.

Log from the failed Linux test bots seem to indicate that
the hints are not processed in time.
See http://shortn/_IjBGHDP2Rp

Bug: 789542,831838
Change-Id: I6b30b1a86dc5d2cdef1cd8274cf5c6088bec8634
Reviewed-on: https://chromium-review.googlesource.com/1134598
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574613}
2 files changed