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

ash/display/mirror_window_controller.h

Index: ash/display/mirror_window_controller.h
diff --git a/ash/display/mirror_window_controller.h b/ash/display/mirror_window_controller.h
index e7b96c78613da4caf3dbaea1afa1070443b113fa..3db86b393de2609b2453a7516732518ba6e1e43e 100644
--- a/ash/display/mirror_window_controller.h
+++ b/ash/display/mirror_window_controller.h
@@ -37,6 +37,11 @@ class MirrorWindowController {
   // 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