Find root scroll layer at tree activation

cc/top_controls_manager_client.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_TOP_CONTROLS_MANAGER_CLIENT_H_
 #define CC_TOP_CONTROLS_MANAGER_CLIENT_H_
 
 namespace cc {
 
 class LayerTreeImpl;
 
 class CC_EXPORT TopControlsManagerClient {
  public:
   virtual void setNeedsRedraw() = 0;
   virtual void setNeedsUpdateDrawProperties() = 0;
-  virtual LayerTreeImpl* activeTree() = 0;
+  virtual bool haveRootScrollLayer() const = 0;
+  virtual float rootScrollLayerTotalScrollY() const = 0;
 
  protected:
   virtual ~TopControlsManagerClient() {}
 };
 
 }  // namespace cc
 
 #endif  // CC_TOP_CONTROLS_MANAGER_CLIENT_H_