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This requires notifying layout_helper_ when a tab is being destroyed,
and TabStripLayoutHelper to step carefully around closing tabs in a few
places, but saves more complexity by not duplicating state between
TabStrip and TabStripLayoutHelper.

Bug: 958173
Change-Id: Iaeafc2391ecac0397947ccc34ddaecf853feb787
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1725271
Commit-Queue: Taylor Bergquist <tbergquist@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683380}