Touchpad scroll latching enabled for Mac 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 df9e29bc9415aa07188c3c3b1a16a47ca3049314..cc232e34ba5ab735fa2ea52492d581b32028ebce 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2725,6 +2725,11 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBegin(
       MainThreadScrollingReason::kNotScrollingOnMain;
   TRACE_EVENT0("cc", "LayerTreeHostImpl::ScrollBegin");
 
+  // On Mac a scroll begin with |inertial_phase| = true happens to handle a
+  // fling.
+  if (scroll_state->is_in_inertial_phase())
+    return FlingScrollBegin();
+
   ClearCurrentlyScrollingLayer();
 
   gfx::Point viewport_point(scroll_state->position_x(),
@@ -2887,7 +2892,7 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollAnimated(
   ScrollStateData scroll_state_data;
   scroll_state_data.position_x = viewport_point.x();
   scroll_state_data.position_y = viewport_point.y();
-  scroll_state_data.is_in_inertial_phase = true;
+  scroll_state_data.is_in_inertial_phase = false;
   ScrollState scroll_state(scroll_state_data);
 
   // ScrollAnimated is used for animated wheel scrolls. We find the first layer
@@ -2925,7 +2930,12 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollAnimated(
     }
   }
   scroll_state.set_is_ending(true);
+  // TODO(Sahel): Once the touchpad scroll latching for Non-mac devices is
+  // impelemented, the current scrolling layer should not get cleared after
+  // each animation (crbug.com/526463).
   ScrollEnd(&scroll_state);
+  ClearCurrentlyScrollingLayer();
+
   return scroll_status;
 }
 
@@ -3279,7 +3289,12 @@ void LayerTreeHostImpl::ScrollEnd(ScrollState* scroll_state) {
 
   DistributeScrollDelta(scroll_state);
   top_controls_manager_->ScrollEnd();
-  ClearCurrentlyScrollingLayer();
+
+  if (scroll_state->is_in_inertial_phase()) {
+    // 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.
+    ClearCurrentlyScrollingLayer();
+  }
 }
 
 InputHandler::ScrollStatus LayerTreeHostImpl::FlingScrollBegin() {
@@ -4093,7 +4108,11 @@ void LayerTreeHostImpl::ScrollOffsetAnimationFinished() {
   // TODO(majidvp): We should pass in the original starting scroll position here
   ScrollStateData scroll_state_data;
   ScrollState scroll_state(scroll_state_data);
+  // TODO(Sahel): Once the touchpad scroll latching for Non-mac devices is
+  // impelemented, the current scrolling layer should not get cleared after
+  // each animation (crbug.com/526463).
   ScrollEnd(&scroll_state);
+  ClearCurrentlyScrollingLayer();
 }
 
 gfx::ScrollOffset LayerTreeHostImpl::GetScrollOffsetForAnimation(