I might be possible to eliminate ash::TabletModeObserver entirely and switch over to a mojom:: observer. It could be added in-process via AddObserver inside of ash and added via mojo from chrome.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c0fe3ce7674e1700f15c4532dd06d9d411f20ae9

commit c0fe3ce7674e1700f15c4532dd06d9d411f20ae9
Author: Elliot Glaysher <erg@chromium.org>
Date: Mon Oct 02 22:52:25 2017

Move ChromeNativeAppWindowViewsAuraAsh to the TabletModeClient interface

Bug: 769098,  756046 
Change-Id: I134d37276c02e38f38e9d3677683912dbceb7b39
Reviewed-on: https://chromium-review.googlesource.com/688475
Commit-Queue: Elliot Glaysher <erg@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505831}
[modify] https://crrev.com/c0fe3ce7674e1700f15c4532dd06d9d411f20ae9/ash/wm/tablet_mode/tablet_mode_controller.cc
[modify] https://crrev.com/c0fe3ce7674e1700f15c4532dd06d9d411f20ae9/ash/wm/tablet_mode/tablet_mode_controller.h
[modify] https://crrev.com/c0fe3ce7674e1700f15c4532dd06d9d411f20ae9/chrome/browser/ui/BUILD.gn
[add] https://crrev.com/c0fe3ce7674e1700f15c4532dd06d9d411f20ae9/chrome/browser/ui/ash/fake_tablet_mode_controller.cc
[add] https://crrev.com/c0fe3ce7674e1700f15c4532dd06d9d411f20ae9/chrome/browser/ui/ash/fake_tablet_mode_controller.h
[modify] https://crrev.com/c0fe3ce7674e1700f15c4532dd06d9d411f20ae9/chrome/browser/ui/ash/tablet_mode_client_unittest.cc
[modify] https://crrev.com/c0fe3ce7674e1700f15c4532dd06d9d411f20ae9/chrome/browser/ui/views/apps/chrome_native_app_window_views_aura_ash.cc
[modify] https://crrev.com/c0fe3ce7674e1700f15c4532dd06d9d411f20ae9/chrome/browser/ui/views/apps/chrome_native_app_window_views_aura_ash.h
[modify] https://crrev.com/c0fe3ce7674e1700f15c4532dd06d9d411f20ae9/chrome/browser/ui/views/apps/chrome_native_app_window_views_aura_ash_browsertest.cc