bento: Group windows by desk in "Move Tab to Window" menu.

Currently the "Move Tab to Window" menu lists browsers across all desks
by usage order, making it unclear as to which desk a browser belongs
to. To help users engage with desks better, this CL groups windows by
desks in this menu.

This CL does the following:
  - Adds ExistingWindowSubMenuModelChromeOS.
  - Adds a factory function to ExistingWindowSubMenuModel which
      returns platform-specific impls.
  - Makes ExistingWindowSubMenuModelChromeOS override
      MenuModel::GetLabelFontListAt()

Test: added
Bug: 1221287
Change-Id: I4b38b37d58d7bfa20401b2603bf492fe45d07b25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3049091


Commit-Queue: Jeremy Chinsen <chinsenj@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Reviewed-by: Emily Shack <emshack@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#908670}