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..a747219aca3aa25618ea06f6b9ec2d8019176c78 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);
+    // Get the current_offset_.y() value for a sanity-check on scrolling

[enne (OOO), 2015/03/24 17:53:21]

This comment seems unnecessary.

[wjmaclean, 2015/03/24 18:21:40]

Don't we want to indicate why we're interested in recording the scroll offsets?
I've edited the comment to make it briefer and indicate current_offset isn't a
member variable, but it can go away entirely if you like.

+    // benchmark metrics. Specifically, we want to make sure
+    // BasicMouseWheelSmoothScrollGesture has proper scroll curves.
+    if (y_offset_did_change && layer_tree_impl()->IsActiveTree()) {
+      TRACE_COUNTER_ID1("gpu", "scroll_offset_y", this->id(),

[enne (OOO), 2015/03/24 17:53:21]

This should be a cc trace, I think?

Should we only do this on the main scrolling layer, or does every scrolling
layer get a trace?

[wjmaclean, 2015/03/24 18:21:40]

Done.
The previous implementation seems to have reported for every scrolling layer
(presumably this is why the id() was previously included).

+paulirish@

Paul, did you just want this trace for the main scrolling layer? (If so, does
that mean both inner/outer viewports for pinch-zoom?)

+                        current_offset.y());
+    }
   }
   if (scrollbar_needs_animation) {
     layer_tree_impl()->set_needs_update_draw_properties();