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This CL adds animation when an ungrouped, unpinned tab is moved, to
mostly facilitate the drag and drop experience. When the TabList's
placeTabElement is called, the method first determines if the
placement is causing a TabElement to move, then identifies which
elements the TabElement is being dragged across, and finally
animates the necessary distances for each TabElement.

Bug: 1082344
Change-Id: Ia0b55f4c8539429969197258d148ce236b29e7e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2215248
Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774469}