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Specifically adds the following:

1. fullscreen for background tabs is processed when the tab is made
active. This matches what clank does.
2. Ensure the callback supplied to the delegate to exit fullscreen is
only applicable to the request it was created for. For example, if the
the delegate is supplied callback C1, fullscreen exits, then fullscreen
enters again with callback C2, callback C1 is no longer applicable.
3. Make the java side have similar logic to (2).

BUG=1142101
TEST=none

Change-Id: Ic28d6cbc761ff8b0f9651c018c258d569b9c556b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2628086Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843644}