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This CL enables dropping a tab on another tab to form a group. It is
behind the TabGroupsAndroidUiImprovements flag.

Bug: 963692
Change-Id: I1cb49a20459972af3c02bdc7c9ebb9480209de29
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1615606Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Commit-Queue: Yue Zhang <yuezhanggg@google.com>
Cr-Commit-Position: refs/heads/master@{#666450}