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Frame observer is registered right away on each frame initialization.

Why:
Sink cache lives on the main task runner, but also accepts sink requests from
the media task runner (media pipeline). So the sinks can be cashed on
2 different threads.
However, frame observers can be registered on the main renderer thread only.
Because of the above, on-demand observer registration (i.e. registering an
observer only when a sink is cached for a frame) is racy: navigation can
happen before the observer is registered.

BUG=836420

Change-Id: I9377201d93ae5c12e4fe05c8f23514b68bfddbe1
Reviewed-on: https://chromium-review.googlesource.com/1050127
Commit-Queue: Olga Sharonova <olka@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Max Morin <maxmorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557342}