Prevent create ScrollbarAnimationController for Mac Overlay Scrollbar

cc/layers/scrollbar_layer_impl_base.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_IMPL_BASE_H_
 #define CC_LAYERS_SCROLLBAR_LAYER_IMPL_BASE_H_
 
 #include "base/containers/flat_set.h"
 #include "base/macros.h"
 #include "cc/cc_export.h"
@@ skipping 43 matching lines @@
     return thumb_thickness_scale_factor_;
   }
   bool SetThumbThicknessScaleFactor(float thumb_thickness_scale_factor);
 
   virtual int ThumbThickness() const = 0;
 
   // TODO(crbug.com/702832): No need for this function once there is element id
   // on overlay scrollbar layers.
   void SetOverlayScrollbarLayerOpacityAnimated(float opacity);
 
+  virtual bool IsSolidColorScrollbarLayerImpl() const;

[aelias_OOO_until_Jul13, 2017/06/02 22:02:23]

How about instead making scrollbars return one of the animator enum types to
specify which one they're compatible with?

[chaopeng, 2017/06/05 14:52:39]

Yes, this would be better.

+
  protected:
   ScrollbarLayerImplBase(LayerTreeImpl* tree_impl,
                          int id,
                          ScrollbarOrientation orientation,
                          bool is_left_side_vertical_scrollbar,
                          bool is_overlay);
   ~ScrollbarLayerImplBase() override;
 
   virtual int ThumbLength() const = 0;
   virtual float TrackLength() const = 0;
@@ skipping 21 matching lines @@
   float vertical_adjust_;
 
   DISALLOW_COPY_AND_ASSIGN(ScrollbarLayerImplBase);
 };
 
 using ScrollbarSet = base::flat_set<ScrollbarLayerImplBase*>;
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_SCROLLBAR_LAYER_IMPL_BASE_H_