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This is currently causing a small number of crashes in a corner case with no known repro (see attached bug).

It may become a more common pattern if extensions are able to create new, singleton grouped tabs, or if grouped tabs can be restored into new singletons.

Bug: 1006708
Change-Id: I8720b021fe8cd3c677eefa7e7f7639672673b10e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1825660Reviewed-by: Taylor Bergquist <tbergquist@chromium.org>
Commit-Queue: Connie Wan <connily@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700026}