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Previously, tabs would always get inserted to the end of the tab group.
This changes it such that if the tab is currently to the left of the
group, it will be inserted at the front of the tab group respecting the
relative order of the selected tabs. If the tabs are to the right of
the group, it will be inserted to the end of the tab group as before
(no behavior change).

This change also allows tabs in a group to be restored to the location
saved at closing time, or the closest location if the group has moved.

Bug: 1030779, 1038362
Change-Id: Id26cdfa2df769bc738d780e48277e603acef42ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1977849
Commit-Queue: Charlene Yan <cyan@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728337}