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This CL adds a few elements to the new menus in incognito and guest
modes:
 - Add a new background subclass that allows to draw themed images
   on a view's background.
 - Add incognito and guest mode art assets.

Design Deck: go/alignmenus

Screenshots:
https://screenshot.googleplex.com/9nCgPZMahh8eALD.png

Bug: 1105763
Change-Id: I2367514aa7285959409e68e3cfd618690beedff1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2388442Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Ramin Halavati <rhalavati@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Commit-Queue: Mariam Ali <alimariam@google.com>
Cr-Commit-Position: refs/heads/master@{#809711}