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This CL adds a tracker that observes a tab's WebContents after it has
been swiped to close on the front-end. The tracker's observer listens
for close events (which will naturally remove the TabElement from the
DOM) or for when a user cancels out of a beforeunload dialog (which will
reset the swipe animation).

Bug: 1059080
Change-Id: Id21355fc8d1b8f1001a10106c801fd514cf35ee2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088410
Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748904}