Move theme part drawing functions only used on Windows to NativeThemeWin

ui/native_theme/native_theme_win.h

Index: ui/native_theme/native_theme_win.h
diff --git a/ui/native_theme/native_theme_win.h b/ui/native_theme/native_theme_win.h
index 8a1c3bf218037060e979776be43eaf714e6bc908..03483650bc57ab4f54b36911a12a9fb7621e5696 100644
--- a/ui/native_theme/native_theme_win.h
+++ b/ui/native_theme/native_theme_win.h
@@ -125,6 +125,11 @@ class NATIVE_THEME_EXPORT NativeThemeWin : public NativeTheme,
   // Update the locally cached set of system colors.
   void UpdateSystemColors();
 
+  // Painting functions that paint to SkCanvas.
+  void PaintMenuSeparator(SkCanvas* canvas, const gfx::Rect& rect) const;
+  void PaintMenuGutter(SkCanvas* canvas, const gfx::Rect& rect) const;
+  void PaintMenuBackground(SkCanvas* canvas, const gfx::Rect& rect) const;
+
   // Paint directly to canvas' HDC.
   void PaintDirect(SkCanvas* canvas,
                    Part part,