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When removing tabs with mouse button, we'd like to know
the next active tab so that we can tailor available_width_for_tabs_.
But as PrepareForCloseAt() is called before the next tab is decided,
we don't know that. So move this logic to RemoveTabAt() where we
know it.
 And move other logic in PrepareCloseTabAt() is moved to CloseTab(),
 which triggered PrepareForCloseAt() via TabStripController::CloseTab().
As TabStripController::CloseTab() was invoked only by
TabStrip::CloseTab(), it's okay to move them to TabStrip::CloseTab().

Bug: 856289
Change-Id: I5eb7036da4214e92a218f20e8c09891ab9a11955
Reviewed-on: https://chromium-review.googlesource.com/1119616
Commit-Queue: Sang Woo Ko <sangwoo108@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572566}