Revert of Mus Demo: Move definition of WindowTreeData into separate files.

services/ui/demo/mus_demo.h

Index: services/ui/demo/mus_demo.h
diff --git a/services/ui/demo/mus_demo.h b/services/ui/demo/mus_demo.h
index 58bb6f53e8c488efc736355cb74e0e2409ac15fa..55112dadbc22d512d8e755a09b804ba9e90ddb24 100644
--- a/services/ui/demo/mus_demo.h
+++ b/services/ui/demo/mus_demo.h
@@ -34,8 +34,6 @@
 
 namespace ui {
 namespace demo {
-
-class WindowTreeData;
 
 // A simple MUS Demo service. This service connects to the service:ui, adds a
 // new window to the root Window, and draws a spinning square in the center of
@@ -104,6 +102,7 @@
   std::unique_ptr<::wm::WMState> wm_state_;
   std::unique_ptr<aura::PropertyConverter> property_converter_;
 
+  class WindowTreeData;
   std::unique_ptr<WindowTreeData> window_tree_data_;
 
   DISALLOW_COPY_AND_ASSIGN(MusDemo);