Mash: observe the right window for immersive fullscreen reveals

In Ash with Single Process Mash enabled, we'd track the root window
instead of the toplevel window. There was no effective bug here since
the root window has the same dimensions as the toplevel window when
immersive is enabled, but this change is technically more correct.

Bug: none
Change-Id: Iecf1534c96c8ab65c331567e8f724b915214ea46
Reviewed-on: https://chromium-review.googlesource.com/c/1477762
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633386}
1 file changed