Enables passing initial properties to top level window creation

ui/aura/window_tree_host_platform.h

Index: ui/aura/window_tree_host_platform.h
diff --git a/ui/aura/window_tree_host_platform.h b/ui/aura/window_tree_host_platform.h
index 0ee8824d2ebb5e36c903699fd3de6745ab084698..773d9f49efabc7dd576807193acfd44f9d50b2d9 100644
--- a/ui/aura/window_tree_host_platform.h
+++ b/ui/aura/window_tree_host_platform.h
@@ -43,6 +43,8 @@ class AURA_EXPORT WindowTreeHostPlatform
   void OnCursorVisibilityChangedNative(bool show) override;
 
  protected:
+  // NOTE: neither of these calls CreateCompositor(); subclasses must call
+  // CreateCompositor() at the appropriate time.

[sadrul, 2016/11/16 18:02:08]

Is this change because the creation of the compositor depends on some of the
attributes on the mus window?

[sky, 2016/11/16 18:45:49]

CreateCompositor() calls window->Init(). window->Init(), for aura-mus, is the
time at which the the window server is asked to create the window. When the
server window is created properties may be provided to help influence initial
configuration of the serverwindow. By delaying the call to CreateCompositor
WindowTreeHostMus now has the ability to set properties on the aura::Window
before Init() is called so that initial properties make there way to
ServerWindow.

   WindowTreeHostPlatform();
   explicit WindowTreeHostPlatform(std::unique_ptr<WindowPort> window_port);