Add a scroll threshold/marker to control hiding/showing top controls.

cc/top_controls_manager.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_H_
 #define CC_TOP_CONTROLS_MANAGER_H_
 
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/weak_ptr.h"
 #include "cc/layer_impl.h"
@@ skipping 50 matching lines @@
   void SetupAnimation(AnimationDirection direction);
   void StartAnimationIfNecessary();
   bool IsAnimationCompleteAtTime(base::TimeTicks time);
 
   TopControlsManagerClient* client_;  // The client manages the lifecycle of
                                       // this.
 
   scoped_ptr<KeyframedFloatAnimationCurve> top_controls_animation_;
   AnimationDirection animation_direction_;
   bool is_overlay_mode_;
-  bool scroll_readjustment_enabled_;
+  bool in_scroll_gesture_;
   float controls_top_offset_;
   float content_top_offset_;
   float top_controls_height_;
   float previous_root_scroll_offset_;
+  float scroll_start_offset_;
 
   DISALLOW_COPY_AND_ASSIGN(TopControlsManager);
 };
 
 }  // namespace cc
 
 #endif  // CC_TOP_CONTROLS_MANAGER_H_