Use the source display's pixel size as a mirror window's size.

ash/display/display_controller.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ASH_DISPLAY_DISPLAY_CONTROLLER_H_
 #define ASH_DISPLAY_DISPLAY_CONTROLLER_H_
 
 #include <map>
 #include <vector>
 
@@ skipping 71 matching lines @@
 };
 
 // DisplayController owns and maintains RootWindows for each attached
 // display, keeping them in sync with display configuration changes.
 class ASH_EXPORT DisplayController : public gfx::DisplayObserver {
  public:
   class ASH_EXPORT Observer {
    public:
     // Invoked when the display configuration change is requested,
     // but before the change is applied to aura/ash.
-    virtual void OnDisplayConfigurationChanging() = 0;
+    virtual void OnDisplayConfigurationChanging() {}
 
     // Invoked when the all display configuration changes
     // have been applied.
     virtual void OnDisplayConfigurationChanged() {};
 
    protected:
     virtual ~Observer() {}
   };
 
   DisplayController();
@@ skipping 168 matching lines @@
   bool in_bootstrap_;
 
   scoped_ptr<internal::FocusActivationStore> focus_activation_store_;
 
   DISALLOW_COPY_AND_ASSIGN(DisplayController);
 };
 
 }  // namespace ash
 
 #endif  // ASH_DISPLAY_DISPLAY_CONTROLLER_H_