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..6522abe578c10d77de1ebc6a717520354a334bcb 100644
--- a/cc/layers/layer_impl.cc
+++ b/cc/layers/layer_impl.cc
@@ -1322,12 +1322,20 @@ 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 evaluate scroll latency

[enne (OOO), 2015/03/25 20:52:04]

At this point, I think this comment doesn't add anything.  It's a trace event
for scroll offset, and so saying it's about evaluating scrolling in traces is
redundant.

[wjmaclean, 2015/03/25 21:00:42]

Removed.

I thought the notion of measuring latency might be worth documenting, but I
guess that's implicit in that all trace events have time associated with them.

+    // via traces.
+    if (y_offset_did_change && layer_tree_impl()->IsActiveTree() &&
+        this == layer_tree_impl()->InnerViewportScrollLayer()) {
+      TRACE_COUNTER_ID1("cc", "scroll_offset_y", this->id(),
+                        current_offset.y());
+    }
   }
   if (scrollbar_needs_animation) {
     layer_tree_impl()->set_needs_update_draw_properties();