Use nine-patch resource for drawing Aura overlay scrollbar thumb.

ui/native_theme/native_theme.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_H_
 #define UI_NATIVE_THEME_NATIVE_THEME_H_
 
 #include "base/macros.h"
 #include "base/observer_list.h"
 #include "build/build_config.h"
 #include "cc/paint/paint_canvas.h"
 #include "third_party/skia/include/core/SkColor.h"
 #include "ui/base/models/menu_separator_types.h"
+#include "ui/gfx/geometry/rect.h"
+#include "ui/gfx/geometry/size.h"
 #include "ui/gfx/native_widget_types.h"
 #include "ui/native_theme/native_theme_export.h"
 
 namespace gfx {
 class Rect;
 class Size;
 }
 
 namespace ui {
 
@@ skipping 236 matching lines @@
   // Paint part during state transition, used for overlay scrollbar state
   // transition animation.
   virtual void PaintStateTransition(cc::PaintCanvas* canvas,
                                     Part part,
                                     State startState,
                                     State endState,
                                     double progress,
                                     const gfx::Rect& rect,
                                     ScrollbarOverlayColorTheme theme) const {}
 
+  virtual bool SupportsNinePatch(Part part) const = 0;

[sadrul, 2017/03/01 15:38:55]

Can you document this?

[bokan, 2017/03/01 16:22:45]

Done.

+  virtual gfx::Size GetNinePatchCanvasSize(Part part) const = 0;
+  virtual gfx::Rect GetNinePatchAperture(Part part) const = 0;
+
   // Supports theme specific colors.
   void SetScrollbarColors(unsigned inactive_color,
                           unsigned active_color,
                           unsigned track_color);
 
   // Colors for GetSystemColor().
   enum ColorId {
     // Windows
     kColorId_WindowBackground,
     // Dialogs
@@ skipping 135 matching lines @@
  private:
   // Observers to notify when the native theme changes.
   base::ObserverList<NativeThemeObserver> native_theme_observers_;
 
   DISALLOW_COPY_AND_ASSIGN(NativeTheme);
 };
 
 }  // namespace ui
 
 #endif  // UI_NATIVE_THEME_NATIVE_THEME_H_