Re-introduce ScrollY trace event.

cc/layers/layer_impl.cc

Index: cc/layers/layer_impl.cc
diff --git a/cc/layers/layer_impl.cc b/cc/layers/layer_impl.cc
index a1a61618720e9b678b4aa577dee5866da683f52d..4b78470bef678bbb263fa94f63e6675173efdf8b 100644
--- a/cc/layers/layer_impl.cc
+++ b/cc/layers/layer_impl.cc
@@ -1322,12 +1322,17 @@ void LayerImpl::SetScrollbarPosition(ScrollbarLayerImplBase* scrollbar_layer,
         scrollbar_layer->SetVisibleToTotalLengthRatio(visible_ratio);
   } else {
     float visible_ratio = clip_rect.height() / scroll_rect.height();
-    scrollbar_needs_animation |=
+    bool y_offset_did_change =
         scrollbar_layer->SetCurrentPos(current_offset.y());
+    scrollbar_needs_animation |= y_offset_did_change;
     scrollbar_needs_animation |=
         scrollbar_layer->SetMaximum(scroll_rect.height() - clip_rect.height());
     scrollbar_needs_animation |=
         scrollbar_layer->SetVisibleToTotalLengthRatio(visible_ratio);
+    // Record the current_offset.y() value so we can make sure
+    // BasicMouseWheelSmoothScrollGesture has proper scroll curves.

[enne (OOO), 2015/03/25 17:21:33]

What is BasicMouseWheelSmoothScrollGesture? I can't find any reference to that
in Chromium.

[wjmaclean, 2015/03/25 20:30:32]

Revised the comment to reflect the rationale given in the associated bug, namely
the desire to evaluate scroll latency via tracing.

+    if (y_offset_did_change && layer_tree_impl()->IsActiveTree())

[wjmaclean, 2015/03/25 20:30:32]

Will limit this to the InnerViewportScrollLayer() as per Paul's suggestion.

+      TRACE_EVENT1("cc", "scroll_offset_y", this->id(), current_offset.y());
   }
   if (scrollbar_needs_animation) {
     layer_tree_impl()->set_needs_update_draw_properties();