Add impl/comp scrolling instrumentation.

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index 9956f4c7359887d8c3fa44f720f6c7177abf780d..ea87e3041b5703329e2ac492b8465715bca56014 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -995,6 +995,18 @@ void LayerTreeHost::ApplyScrollAndScale(const ScrollAndScaleSet& info) {
   if (!root_layer_.get())
     return;
 
+  const bool has_scrolls =
+      !info.scrolls.empty() || info.page_scale_delta != 1.0f;
+
+  if (has_scrolls) {
+    // We use async events here because we want to conditionally trace. This
+    // is too spammy otherwise.
+    TRACE_EVENT_ASYNC_BEGIN0(

[dsinclair, 2013/08/08 17:30:14]

Why not TRACE_EVENT_BEGIN0 and END0 below?

+        "impl-scroll",
+        "LayerTreeHost::ApplyScrollAndScale",
+        this);
+  }
+
   gfx::Vector2d root_scroll_delta;
   Layer* root_scroll_layer = FindFirstScrollableLayer(root_layer_.get());
 
@@ -1027,6 +1039,13 @@ void LayerTreeHost::ApplyScrollAndScale(const ScrollAndScaleSet& info) {
     ApplyPageScaleDeltaFromImplSide(info.page_scale_delta);
     client_->ApplyScrollAndScale(root_scroll_delta, info.page_scale_delta);
   }
+
+  if (has_scrolls) {
+    TRACE_EVENT_ASYNC_END0(
+        "impl-scroll",
+        "LayerTreeHost::ApplyScrollAndScale",
+        this);
+  }
 }
 
 void LayerTreeHost::StartRateLimiter(WebKit::WebGraphicsContext3D* context3d) {