[iOS][TabStrip] Add support for parent group items

This CL updates the TabStripViewController to support a hierarchy of
items instead of a flat sequence. Items associated with TabGroups are
now the parent of items associated with WebStates in that TabGroup.
The TabStripMediator and drag&drop are updated accordingly.

Fixed: 329091020
Change-Id: I63dbbd56eec6d7f49af44ce7e2ea4c9590e5bd34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5374773


Commit-Queue: Quentin Pubert <qpubert@google.com>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#12811061}