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This CL adds animations to grouped tabs and entire groups being dragged.
It reuses most of the already implemented methods of animating tabs
because tab groups are shrunken to the width of one TabElement when
being dragged.

The CL also removes a previous |.remove()| call when placing a
TabGroupElement as the removal and inserting of the TabGroupElement
while animating caused some performance issues.

Bug: 1082344
Change-Id: I04c757b4d2d7f919a3021491e559e020f7e1010d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2252036
Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780391}