Adding tracing to measure synthetic smooth scrolling correctness

cc/layer_tree_host_impl.cc

Index: cc/layer_tree_host_impl.cc
diff --git a/cc/layer_tree_host_impl.cc b/cc/layer_tree_host_impl.cc
index e8ff61f8713282f897df59dd3571e1df1315dee9..c994d7b6faa52153ef52e6573003c9db4a614672 100644
--- a/cc/layer_tree_host_impl.cc
+++ b/cc/layer_tree_host_impl.cc
@@ -800,6 +800,13 @@ void LayerTreeHostImpl::drawLayers(const FrameData& frame)
     // RenderWidget.
     m_fpsCounter->markBeginningOfFrame(base::TimeTicks::Now());
 
+    // Get the TranslateY value for a sanity-check on scrolling benchmark
+    // metrics. Specifically, we want to make sure
+    // BasicMouseWheelSmoothScrollGesture has proper scroll curves.
+    TRACE_COUNTER_ID1("gpu", "frame_counter_translate_y", this,

[nduca, 2012/11/27 23:50:27]

maybe root_layer_y_translate?

[shawnsingh, 2012/11/27 23:59:58]

I also like nduca's proposed rename.

What assumptions are we allowed to make about the transform that the root
scrollLayer will ever have?  Are we certain it will only ever have translation? 
If this layer has more complicated transforms, are you sure this is the value
you want to trace?

assuming you know that's the value you want, it LGTM.

+                      m_rootScrollLayerImpl->drawTransform().matrix().getDouble(

[jamesr, 2012/11/28 00:02:16]

cc::LayerImpl has a scrollOffset, can't you use that instead of looking at the
draw transform?

+                          1, 3));
+
     if (m_settings.showDebugRects())
         m_debugRectHistory->saveDebugRectsForCurrentFrame(m_rootLayerImpl.get(), *frame.renderSurfaceLayerList, frame.occludingScreenSpaceRects, frame.nonOccludingScreenSpaceRects, settings());