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

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 <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 2920 matching lines @@
 
   gfx::ScrollOffset current_offset =
       scroll_tree.current_scroll_offset(scroll_node->owning_layer_id);
   gfx::ScrollOffset target_offset = scroll_tree.ClampScrollOffsetToLimits(
       current_offset + gfx::ScrollOffset(delta), scroll_node);
   DCHECK_EQ(
       ElementId(
           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();
 
   return true;
 }
 
 InputHandler::ScrollStatus LayerTreeHostImpl::ScrollAnimated(
     const gfx::Point& viewport_point,
     const gfx::Vector2dF& scroll_delta,
     base::TimeDelta delayed_by) {
@@ skipping 1350 matching lines @@
 
 void LayerTreeHostImpl::ShowScrollbarsForImplScroll(ElementId element_id) {
   if (!element_id)
     return;
   if (ScrollbarAnimationController* animation_controller =
           ScrollbarAnimationControllerForElementId(element_id))
     animation_controller->DidScrollUpdate();
 }
 
 }  // namespace cc