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

cc/layers/painted_overlay_scrollbar_layer.h

Index: cc/layers/painted_overlay_scrollbar_layer.h
diff --git a/cc/layers/painted_overlay_scrollbar_layer.h b/cc/layers/painted_overlay_scrollbar_layer.h
new file mode 100644
index 0000000000000000000000000000000000000000..6888bec7ce6217d53f59b744d5d6ffe6bafafa9d
--- /dev/null
+++ b/cc/layers/painted_overlay_scrollbar_layer.h
@@ -0,0 +1,77 @@
+// Copyright 2016 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 CC_LAYERS_PAINTED_OVERLAY_SCROLLBAR_LAYER_H_
+#define CC_LAYERS_PAINTED_OVERLAY_SCROLLBAR_LAYER_H_
+
+#include "base/macros.h"
+#include "cc/base/cc_export.h"
+#include "cc/input/scrollbar.h"
+#include "cc/layers/layer.h"
+#include "cc/layers/scrollbar_layer_interface.h"
+#include "cc/layers/scrollbar_theme_painter.h"
+#include "cc/resources/scoped_ui_resource.h"
+
+namespace cc {
+
+class CC_EXPORT PaintedOverlayScrollbarLayer : public ScrollbarLayerInterface,
+                                               public Layer {
+ public:
+  std::unique_ptr<LayerImpl> CreateLayerImpl(LayerTreeImpl* tree_impl) override;
+
+  static scoped_refptr<PaintedOverlayScrollbarLayer> Create(
+      std::unique_ptr<Scrollbar> scrollbar,
+      int scroll_layer_id);
+
+  bool OpacityCanAnimateOnImplThread() const override;
+  bool AlwaysUseActiveTreeOpacity() const override;
+  ScrollbarLayerInterface* ToScrollbarLayer() override;
+
+  // ScrollbarLayerInterface
+  int ScrollLayerId() const override;
+  void SetScrollLayer(int layer_id) override;
+  ScrollbarOrientation orientation() const override;
+
+  // Layer interface
+  bool Update() override;
+  void SetLayerTreeHost(LayerTreeHost* host) override;
+  void PushPropertiesTo(LayerImpl* layer) override;
+
+ protected:
+  PaintedOverlayScrollbarLayer(std::unique_ptr<Scrollbar> scrollbar,
+                               int scroll_layer_id);
+  ~PaintedOverlayScrollbarLayer() override;
+
+ private:
+  gfx::Rect OriginThumbRectForPainting() const;
+
+  template <typename T>
+  bool UpdateProperty(T value, T* prop) {
+    if (*prop == value)
+      return false;
+    *prop = value;
+    SetNeedsPushProperties();
+    return true;
+  }
+
+  bool PaintThumbIfNeeded();
+
+  std::unique_ptr<Scrollbar> scrollbar_;
+  int scroll_layer_id_;
+
+  int thumb_thickness_;
+  int thumb_length_;
+  gfx::Point location_;
+  gfx::Rect track_rect_;
+
+  gfx::Rect aperture_;
+
+  std::unique_ptr<ScopedUIResource> thumb_resource_;
+
+  DISALLOW_COPY_AND_ASSIGN(PaintedOverlayScrollbarLayer);
+};
+
+}  // namespace cc
+
+#endif  // CC_LAYERS_PAINTED_OVERLAY_SCROLLBAR_LAYER_H_