mash: Remove WmRootWindowControllerObserver.

ui/display/display_observer.h

Index: ui/display/display_observer.h
diff --git a/ui/display/display_observer.h b/ui/display/display_observer.h
index a41e7068102b12b5a8a1413d171ffadeca7dfbfe..92afb1a40699d4d7745dce8d693e411d9b827810 100644
--- a/ui/display/display_observer.h
+++ b/ui/display/display_observer.h
@@ -27,15 +27,15 @@ class DISPLAY_EXPORT DisplayObserver {
   };
 
   // Called when |new_display| has been added.
-  virtual void OnDisplayAdded(const Display& new_display) = 0;
+  virtual void OnDisplayAdded(const Display& new_display);
 
   // Called when |old_display| has been removed.
-  virtual void OnDisplayRemoved(const Display& old_display) = 0;
+  virtual void OnDisplayRemoved(const Display& old_display);
 
   // Called when a |display| has one or more metrics changed. |changed_metrics|
   // will contain the information about the change, see |DisplayMetric|.
   virtual void OnDisplayMetricsChanged(const Display& display,
-                                       uint32_t changed_metrics) = 0;
+                                       uint32_t changed_metrics);
 
  protected:
   virtual ~DisplayObserver();