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This makes it so that TabDragController::UpdateGroupForDraggedTabs does
only what its name suggests (updates the group membership of the dragged
tabs) without doing anything extra (lots of tab movement).

The assumption is that tabs are moved to their destination before
UpdateGroupForDraggedTabs is called. This assumption is commented here:
https://source.chromium.org/chromium/chromium/src/+/master:chrome/browser/ui/views/tabs/tab_drag_controller.h;l=527
And it still holds because of this call here:
https://source.chromium.org/chromium/chromium/src/+/master:chrome/browser/ui/views/tabs/tab_drag_controller.cc;l=1034

Most of the deleted movements in UpdateGroupForDraggedTabs were added
because functions like RemoveFromGroup() could potentially shift tabs
around. However, by using MoveAndSetGroup instead, the tabs don't
actually move as long as the above assumption holds.

Tested manually and via interactive UI tests:
https://source.chromium.org/chromium/chromium/src/+/master:chrome/browser/ui/views/tabs/tab_drag_controller_interactive_uitest.cc;l=698

Bug: 1163845
Change-Id: I447c66a10c26a20a10d08979cd47cee4906d9b71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622324Reviewed-by: Charlene Yan <cyan@chromium.org>
Commit-Queue: Connie Wan <connily@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843162}