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

ash/display/mirror_window_controller.h

 // Copyright (c) 2013 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_MIRROR_WINDOW_CONTROLLER_H_
 #define ASH_DISPLAY_MIRROR_WINDOW_CONTROLLER_H_
 
 #include "ash/ash_export.h"
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 19 matching lines @@
 // is typically drawn by the window system.
 class MirrorWindowController {
  public:
   MirrorWindowController();
   ~MirrorWindowController();
 
   // Updates the root window's bounds using |display_info|.
   // Creates the new root window if one doesn't exist.
   void UpdateWindow(const DisplayInfo& display_info);
 
+  // Same as above, but using existing display info
+  // for the mirrored display.
+  void UpdateWindow();
+
+  // Close the mirror window.
   void Close();
 
   // Updates the mirrored cursor location,shape and
   // visibility.
   void UpdateCursorLocation();
   void SetMirroredCursor(gfx::NativeCursor cursor);
   void SetMirroredCursorVisibility(bool visible);
 
  private:
   friend class test::MirrorWindowTestApi;
 
   int current_cursor_type_;
   aura::Window* cursor_window_;  // owned by root window.
   scoped_ptr<aura::RootWindow> root_window_;
   scoped_ptr<CursorWindowDelegate> cursor_window_delegate_;
   gfx::Point hot_point_;
 
   DISALLOW_COPY_AND_ASSIGN(MirrorWindowController);
 };
 
 }  // namespace internal
 }  // namespace ash
 
 #endif  // ASH_DISPLAY_MIRROR_WINDOW_CONTROLLER_H_