Wheel scroll latching enabled behind flag.

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 e6db70e6cbf14ee4bb5643da1ca6ed8efd3569dc..409f440e8c33a759957bf36654326e08dd5a7629 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2684,8 +2684,13 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBegin(
   ScrollStatus scroll_status;
   scroll_status.main_thread_scrolling_reasons =
       MainThreadScrollingReason::kNotScrollingOnMain;
+
   TRACE_EVENT0("cc", "LayerTreeHostImpl::ScrollBegin");
 
+  // In Mac a scroll begin with inertial_phase = true happens to handle a fling.

[tdresser, 2016/07/22 15:43:15]

In Mac -> On Mac

+  if (scroll_state->is_in_inertial_phase())
+    return FlingScrollBegin();
+
   ClearCurrentlyScrollingLayer();
 
   gfx::Point viewport_point(scroll_state->position_x(),
@@ -3238,7 +3243,13 @@ void LayerTreeHostImpl::ScrollEnd(ScrollState* scroll_state) {
 
   DistributeScrollDelta(scroll_state);
   top_controls_manager_->ScrollEnd();
-  ClearCurrentlyScrollingLayer();
+
+  if (scroll_state->is_in_inertial_phase()) {
+    // There might be a fling after the scroll end. In those cases,
+    // don't clear the currently scrolling layer to avoid a new hit test for the
+    // fling.

[tdresser, 2016/07/22 15:43:15]

This comment is a bit confusing, since it's talking about not clearing the
scrolling layer in some cases, but the code clears the scrolling layer in some
cases.

You can probably word the comment as "Only clear the currently scrolling layer
if we know the scroll is done. A non-inertial scroll end could be followed by an
inertial scroll." or something like that.

+    ClearCurrentlyScrollingLayer();
+  }
 }
 
 InputHandler::ScrollStatus LayerTreeHostImpl::FlingScrollBegin() {