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* Make TabGridDialog component own a TabSelectionEditor to support
  ungrouping through selection mode for both dialog in tab switcher and
  dialog from strip.
* Add a TabSelectionEditorLayoutPositionProvider to specify the
  position to show selection editor so that it always shows on
  TabGridDialog.
* Add strings used in tab grid dialog selection mode ungrouping.

Bug: 1000381
Change-Id: I78fb56fcfe3ae058f8a6b921a72326e9358f7175
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1829239
Commit-Queue: Yue Zhang <yuezhanggg@chromium.org>
Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702430}