Make it possible for the scroll offset delegate to intercept fling.

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 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.
 
 #include "cc/trees/layer_tree_host_impl.h"
 
 #include <algorithm>
 
 #include "base/basictypes.h"
 #include "base/json/json_writer.h"
@@ skipping 2054 matching lines @@
 }
 
 void LayerTreeHostImpl::ScrollEnd() {
   if (top_controls_manager_)
     top_controls_manager_->ScrollEnd();
   ClearCurrentlyScrollingLayer();
   StartScrollbarAnimation();
 }
 
 InputHandler::ScrollStatus LayerTreeHostImpl::FlingScrollBegin() {
-  if (active_tree_->CurrentlyScrollingLayer())
-    return ScrollStarted;
+  if (!active_tree_->CurrentlyScrollingLayer())
+    return ScrollIgnored;
 
-  return ScrollIgnored;
+  if (settings_.ignore_root_layer_flings &&
+      active_tree_->CurrentlyScrollingLayer() ==
+          active_tree_->RootScrollLayer()) {
+    ClearCurrentlyScrollingLayer();
+    return ScrollIgnored;
+  }
+
+  return ScrollStarted;
 }
 
 void LayerTreeHostImpl::NotifyCurrentFlingVelocity(gfx::Vector2dF velocity) {
   current_fling_velocity_ = velocity;
 }
 
 void LayerTreeHostImpl::PinchGestureBegin() {
   pinch_gesture_active_ = true;
   previous_pinch_anchor_ = gfx::Point();
   client_->RenewTreePriority();
@@ skipping 318 matching lines @@
   if (LayerTreeDebugState::Equal(debug_state_, new_debug_state))
     return;
   if (debug_state_.continuous_painting != new_debug_state.continuous_painting)
     paint_time_counter_->ClearHistory();
 
   debug_state_ = new_debug_state;
   SetFullRootLayerDamage();
 }
 
 }  // namespace cc