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While the typical case is EnterFullscreenModeForTab is not called
multiple times in a row, there may be some corner cases with oopif
that result in it being called consecutively. This ensure the
delegate is only notified once.

BUG=1142101
TEST=FullscreenBrowserTest.DelegateNotCalledMoreThanOnce

Change-Id: Ifa094e53b714f78fe61a0d5dd46a33f08c436e30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2630726
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843788}