Mus Demo: Demonstrate external window mode

ui/aura/window_tree_host.cc

Index: ui/aura/window_tree_host.cc
diff --git a/ui/aura/window_tree_host.cc b/ui/aura/window_tree_host.cc
index 039ec6daecec959a69aafabba001c85f4b746c5a..ffb086cd856d2a6d8efcfe1f0f33c8d41c122162 100644
--- a/ui/aura/window_tree_host.cc
+++ b/ui/aura/window_tree_host.cc
@@ -37,7 +37,12 @@ const char kWindowTreeHostForAcceleratedWidget[] =
 float GetDeviceScaleFactorFromDisplay(Window* window) {
   display::Display display =
       display::Screen::GetScreen()->GetDisplayNearestWindow(window);
+// TODO: This assertion fails in external mode because no display is available

[kylechar, 2017/02/21 15:15:11]

Removing this DCHECK shouldn't be necessary. You'll need to fix the underlying
problem that display::Display objects are not sent to clients in external mode
yet. That will be easier to fix after
https://codereview.chromium.org/2692863009/ lands.

[fwang, 2017/02/21 17:30:19]

OK, as discussed on IRC, the next patch will make MusExternal create a fake
display instead, so that this hack won't be necessary.

+// when the window tree is embedded. Come back to this once bug 693081 is
+// fixed.
+#if !defined(USE_OZONE) || defined(OS_CHROMEOS)
   DCHECK(display.is_valid());
+#endif
   return display.device_scale_factor();
 }