[Tab Groups] Dragging a tab in a tab group to remove from tab group.

Only handles the single select case. Dragging a tab from a tab group to
swap places with a tab not in the tab group will make the tab leave the
tab group.

Still needs interactive UI tests for the following scenarios:
(Manually tested)
- Dragging a tab left out of the tab group with no tab to the left/right
- Dragging a tab left out of the tab group with a tab to the left/right
- Dragging a single tab (where there is only one tab in the group)
- Dragging a tab in a tab group stays in group when consecutive

Bug: 905491
Change-Id: Iddb701c588e84bac054ddb9f6ed0efdba55e018d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1674824
Commit-Queue: Charlene Yan <cyan@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672878}