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

cc/top_controls_manager.h

Index: cc/top_controls_manager.h
diff --git a/cc/top_controls_manager.h b/cc/top_controls_manager.h
index 6ecd352e03c9c7800398c22ed0c7f7e0b110d3f5..a6900038625d1a38964a44812612dfa8894a80b4 100644
--- a/cc/top_controls_manager.h
+++ b/cc/top_controls_manager.h
@@ -68,11 +68,12 @@ class CC_EXPORT TopControlsManager {
   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);
 };