I had a chat with jbauman@. Mus probably still needs its own version of DisplayClient. I'm still going to refactor things out to a DisplayCompositor class, and make TopLevelDisplayClient more like OnscreenDisplayClient for consistency.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4ae9b136bbf91f081db79870a0ceee7d88b78464

commit 4ae9b136bbf91f081db79870a0ceee7d88b78464
Author: fsamuel <fsamuel@chromium.org>
Date: Sat May 14 03:00:07 2016

Pull parts of TopLevelDisplayClient into DisplayCompositor

This CL starts to expose the beginnings of the API DisplayCompositor
should have:

SubmitCompositorFrame
RequestCopyOfOutput

TODO:

Hittest

TopLevelDisplayClient becomes a very thin implementation of
cc::DisplayClient and starts to resemble OnscreenDisplayClient.

BUG= 610937 ,  610935 

Review-Url: https://codereview.chromium.org/1976663003
Cr-Commit-Position: refs/heads/master@{#393731}

[modify] https://crrev.com/4ae9b136bbf91f081db79870a0ceee7d88b78464/components/mus/surfaces/BUILD.gn
[add] https://crrev.com/4ae9b136bbf91f081db79870a0ceee7d88b78464/components/mus/surfaces/display_compositor.cc
[add] https://crrev.com/4ae9b136bbf91f081db79870a0ceee7d88b78464/components/mus/surfaces/display_compositor.h
[modify] https://crrev.com/4ae9b136bbf91f081db79870a0ceee7d88b78464/components/mus/surfaces/top_level_display_client.cc
[modify] https://crrev.com/4ae9b136bbf91f081db79870a0ceee7d88b78464/components/mus/surfaces/top_level_display_client.h
[modify] https://crrev.com/4ae9b136bbf91f081db79870a0ceee7d88b78464/components/mus/ws/platform_display.cc
[modify] https://crrev.com/4ae9b136bbf91f081db79870a0ceee7d88b78464/components/mus/ws/platform_display.h
[modify] https://crrev.com/4ae9b136bbf91f081db79870a0ceee7d88b78464/components/mus/ws/server_window_surface_manager.cc

fsamuel@ finished?

No, not even close :P

I think it's fair to call this bug finished at this point. Mus uses InProcessContextProvider now, and Mus has a lot less Mus-only code. There's a lot more code sharing now.