Make LayerScrollOffsetDelegate updates consistent.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index f11911aa91d64b012cb488aa0d0d029fd91bbe50..60118dd019115c597c9034e90f2a7dbac4980dfa 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2643,6 +2643,11 @@ void LayerTreeHostImpl::PinchGestureUpdate(float magnify_delta,
 
   TRACE_EVENT0("cc", "LayerTreeHostImpl::PinchGestureUpdate");
 
+  // For a moment the scroll offset ends up being outside of the max range. This
+  // confuses the delegate so we switch it off till after we're done processing
+  // the pinch update.
+  active_tree_->SetRootLayerScrollOffsetDelegate(NULL);
+
   // Keep the center-of-pinch anchor specified by (x, y) in a stable
   // position over the course of the magnify.
   float page_scale_delta = active_tree_->page_scale_delta();
@@ -2676,6 +2681,9 @@ void LayerTreeHostImpl::PinchGestureUpdate(float magnify_delta,
     InnerViewportScrollLayer()->ClampScrollToMaxScrollOffset();
   }
 
+  active_tree_->SetRootLayerScrollOffsetDelegate(
+      root_layer_scroll_offset_delegate_);
+
   client_->SetNeedsCommitOnImplThread();
   SetNeedsRedraw();
   client_->RenewTreePriority();