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This adds a feature flag to use the preloaded data to make
"high engagement" decisions and therefore control autoplay.
There is also fixes a bug in our SetHighMediaEngagement code
where a navigation on the same origin does not properly set
the high engagement bit on the Frame.

BUG=787464

Change-Id: I2a85e6f60cbe58c4d4adcf54c344d8a65705cb77
Reviewed-on: https://chromium-review.googlesource.com/787311
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523741}