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This CL adds temporary checkboxes to the bottom right of the tab strip
to allow for easy testing of 4 modes:

1) Most Recently Used ordering with auto-closing tab strip
2) Most Recently Used ordering when tab strip is closed manually
3) Default ordering with auto-closing tab strip
4) Default ordering with no auto-closing tab strip

This CL also brings back drag-and-drop for unpinned tabs for cases where
MRU is disabled.

Bug: 989131, 1005554, 1005565
Change-Id: Iae066c2d85da712ecae29c040468cf67b31fd0d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1867477
Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708018}