Refactor cc scrollbar layers to separate solid-color vs desktop.

cc/layers/scrollbar_layer_interface.h

+// Copyright 2013 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_SCROLLBAR_LAYER_INTERFACE_H_
+#define CC_LAYERS_SCROLLBAR_LAYER_INTERFACE_H_
+
+#include "cc/input/scrollbar.h"
+
+namespace cc {
+
+class ScrollbarLayerInterface {
+ public:
+  virtual int scroll_layer_id() const = 0;
+  virtual void SetScrollLayerId(int id) = 0;
+
+  virtual ScrollbarOrientation orientation() const = 0;
+
+ protected:
+  ScrollbarLayerInterface() {}
+  virtual ~ScrollbarLayerInterface() {}
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(ScrollbarLayerInterface);
+};
+
+}  // namespace cc
+
+#endif  // CC_LAYERS_SCROLLBAR_LAYER_INTERFACE_H_