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

ash/display/display_manager.h

Index: ash/display/display_manager.h
diff --git a/ash/display/display_manager.h b/ash/display/display_manager.h
index c7b47a8fd6b7750b468bf6564df5dd8e1fc3b170..5591b5c0a0a628cbe68b2567f0ae62db39192b08 100644
--- a/ash/display/display_manager.h
+++ b/ash/display/display_manager.h
@@ -143,7 +143,7 @@ class ASH_EXPORT DisplayManager :
 
   // Returns the mirroring status.
   bool IsMirrored() const;
-  int64 mirrored_display_id() const { return mirrored_display_.id(); }
+  const gfx::Display& mirrored_display() const { return mirrored_display_; }
 
   // Returns the display object nearest given |window|.
   const gfx::Display& GetDisplayNearestPoint(
@@ -210,6 +210,10 @@ class ASH_EXPORT DisplayManager :
   gfx::Display& FindDisplayForRootWindow(const aura::RootWindow* root);
   gfx::Display& FindDisplayForId(int64 id);
 
+  // Add the mirror display's display info if the software based
+  // mirroring is in use.
+  void AddMirrorDisplayInfoIfAny(std::vector<DisplayInfo>* display_info_list);
+
   // Refer to |CreateDisplayFromSpec| API for the format of |spec|.
   void AddDisplayFromSpec(const std::string& spec);