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This CL hides tabs and tab groups when they are dragged out of tab
strip to indicate that the tab will leave the current window when
dropped. The elements must be hidden and not removed from the DOM
because once the elements are removed from the DOM, the drag events no
longer get bubbled to its parent.

Bug: 1081905
Change-Id: Ia4d52280362c320648e7cd105e333c62bfb723ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2285067
Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786036}