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Grouping was originally causing two tab movements: one to unpin the tab,
then another to group. Reducing this to a single movement reduces the
potential for race conditions when rapidly pinning and grouping/unpinning
tabs.

Tested manually and via existing unit tests:
https://source.chromium.org/chromium/chromium/src/+/master:chrome/browser/ui/tabs/tab_strip_model_unittest.cc;l=3175

Bug: 1163504
Change-Id: Id2dd3a682afe0d6c8798a7f8f287c5431cd22805
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2613520Reviewed-by: Charlene Yan <cyan@chromium.org>
Commit-Queue: Connie Wan <connily@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841212}