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This CL sets up the ability to drag a tab from one WebUI tab strip and
drop it into another WebUI tab strip. The CL currently only places the
dropped tab to the end of the tab strip, and does not set the index
or the pinned state of the tab.

Future CLs will handle the UI for dropping a tab specifically into
place, and visually differentiating the drops that are allowed and
disallowed.

This implementation currently still uses the Extensions API for
moving tabs between windows, and the Extensions API already checks to
make sure the tab can be legally moved, taking into account
multiple profiles and incognito windows.

Bug: 1005560
Change-Id: I7da09a561144d911660f068f76919efe72f3638a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2020609
Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736060}