Move theme part drawing functions only used on Windows to NativeThemeWin

ui/native_theme/native_theme_win.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_NATIVE_THEME_NATIVE_THEME_WIN_H_
 #define UI_NATIVE_THEME_NATIVE_THEME_WIN_H_
 
 // A wrapper class for working with custom XP/Vista themes provided in
 // uxtheme.dll.  This is a singleton class that can be grabbed using
 // NativeThemeWin::instance().
@@ skipping 107 matching lines @@
   NativeThemeWin();
   ~NativeThemeWin() override;
 
  private:
   // gfx::SysColorChangeListener implementation:
   void OnSysColorChange() override;
 
   // 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,
                    State state,
                    const gfx::Rect& rect,
                    const ExtraParams& extra) const;
 
   // Create a temporary HDC, paint to that, clean up the alpha values in the
   // temporary HDC, and then blit the result to canvas.  This is to work around
   // the fact that Windows XP and some classic themes give bogus alpha values.
@@ skipping 226 matching lines @@
 
   // Is |is_using_high_contrast_| valid?
   mutable bool is_using_high_contrast_valid_;
 
   DISALLOW_COPY_AND_ASSIGN(NativeThemeWin);
 };
 
 }  // namespace ui
 
 #endif  // UI_NATIVE_THEME_NATIVE_THEME_WIN_H_