Add a function to layout.h for getting ScaleFactor of the native view.

ui/base/layout.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 UI_BASE_LAYOUT_H_
 #define UI_BASE_LAYOUT_H_
 
 #include <vector>
 
 #include "build/build_config.h"
 #include "ui/base/ui_export.h"
+#include "ui/gfx/native_widget_types.h"
 
 namespace ui {
 
 enum DisplayLayout {
   // Layout optimized for ASH.  This enum value should go away as soon as
   // LAYOUT_DESKTOP and LAYOUT_ASH are the same.
   LAYOUT_ASH,
 
   // The typical layout for e.g. Windows, Mac and Linux.
   LAYOUT_DESKTOP,
@@ skipping 20 matching lines @@
 };
 
 // Returns the float scale value for |scale_factor|.
 UI_EXPORT float GetScaleFactorScale(ScaleFactor scale_factor);
 
 // Returns the ScaleFactor which most closely matches |scale|.
 // Converting from float to ScaleFactor is inefficient and should be done as
 // little as possible.
 UI_EXPORT ScaleFactor GetScaleFactorFromScale(float scale);
 
+// Returns the ScaleFactor used by |view|.
+UI_EXPORT ScaleFactor GetScaleFactorForNativeView(gfx::NativeView view);
+
 #if defined(OS_MACOSX)
 
 // Returns a vector with the scale factors which are supported by this
 // platform.
 // Only required on Mac so far.
 UI_EXPORT std::vector<ScaleFactor> GetSupportedScaleFactors();
 
 namespace test {
 
 UI_EXPORT void SetSupportedScaleFactors(
     const std::vector<ScaleFactor>& scale_factors);
 
 }  // namespace test
 
 #endif
 
 }  // namespace ui
 
 #endif  // UI_BASE_LAYOUT_H_