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

ash/display/root_window_transformers.h

Index: ash/display/root_window_transformers.h
diff --git a/ash/display/root_window_transformers.h b/ash/display/root_window_transformers.h
new file mode 100644
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+++ b/ash/display/root_window_transformers.h
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+// 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_ROOT_WINDOW_TRANSFORMERS_H_
+#define ASH_DISPLAY_ROOT_WINDOW_TRANSFORMERS_H_
+
+#include "ash/ash_export.h"
+
+namespace aura {
+class RootWindow;
+class RootWindowTransformer;
+}
+
+namespace gfx {
+class Display;
+class Transform;
+}
+
+namespace ash {
+namespace internal {
+class DisplayInfo;
+
+ASH_EXPORT aura::RootWindowTransformer* CreateRootWindowTransformerForDisplay(
+    aura::RootWindow* root,
+    const gfx::Display& display);
+
+// Creates a RootWindowTransformers for mirror root window.
+// |source_display_info| specifies the display being mirrored,
+// and |mirror_display_info| specifies the display used to
+// mirror the content.
+ASH_EXPORT aura::RootWindowTransformer*
+CreateRootWindowTransformerForMirroredDisplay(
+    const DisplayInfo& source_display_info,
+    const DisplayInfo& mirror_display_info);
+
+}  // namespace internal
+}  // namespace ash
+
+#endif  // ASH_DISPLAY_ROOT_WINDOW_TRANSFORMERS_H_