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See bug for details on the issues.

Issue #1: When dragging multiple tabs to the left, reverting doesn't return them to the correct position. Was happening even without groups.
- Fixed by adjusting the target index of revert to account for unreverted tabs.

Issue #2: Intermittent crashes.
- Seems to be fixed by crrev.com/c/2020908

Issue #3: Header is not returned to the initial view position.
- Fixed by adding a call to MoveTabGroup(), which is only needed when dragging within the same tabstrip (otherwise, a new group is being created and positioned correctly).

Issue #4: Selection is not restored correctly.
- Fixed by actively setting the index of group headers to kNoTab, so that checks in RestoreInitialSelection() correctly ignore group headers.

Note: The change at line 657 is to work with line 1656, which is assuming that source_model_index is explicitly set to kNoTab.

Bug: 1047761
Change-Id: I18af6a66d4e7788ed90c9be1f27556fc0018c6c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036362
Commit-Queue: Connie Wan <connily@chromium.org>
Reviewed-by: Charlene Yan <cyan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738296}