Reduce composited smooth scroll latency by a frame (also fixes latency UMAs).

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 62e587ce860fceb390dcb40e31992f43b843aa36..6572c72149d60f3f2f4c0bd66a9e8241a7e2adf5 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2938,8 +2938,14 @@ bool LayerTreeHostImpl::ScrollAnimationCreate(ScrollNode* scroll_node,
           active_tree()->LayerById(scroll_node->owning_layer_id)->element_id()),
       scroll_node->element_id);
 
+  // Start the animation one full frame in. Without any offset, the animation
+  // doesn't start until next frame, increasing latency, and preventing our
+  // input latency tracking architecture from working.
+  base::TimeDelta animation_start_offset = CurrentBeginFrameArgs().interval;
+
   mutator_host_->ImplOnlyScrollAnimationCreate(
-      scroll_node->element_id, target_offset, current_offset, delayed_by);
+      scroll_node->element_id, target_offset, current_offset, delayed_by,
+      animation_start_offset);
 
   SetNeedsOneBeginImplFrame();