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 cdb5f61a82c3866d818fa4cf8ddcefbb6fe3e788..644acc8e1a077053fd59df03a7af902ebada4bcd 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -965,6 +965,16 @@ void LayerTreeHost::ApplyScrollAndScale(const ScrollAndScaleSet& info) {
   if (!root_layer_.get())
     return;
 
+  const bool has_scrolls = !info.scrolls.empty();

[enne (OOO), 2013/07/29 17:46:31]

...or page scale delta != 1?

+  if (has_scrolls) {
+    // We use async events here because we want to conditionally trace. This
+    // is too spammy otherwise.
+    TRACE_EVENT_ASYNC_BEGIN0(
+        "impl-scroll",
+        "LayerTreeHost::ApplyScrollAndScale",
+        this);
+  }
+
   Layer* root_scroll_layer = FindFirstScrollableLayer(root_layer_.get());
   gfx::Vector2d root_scroll_delta;
 
@@ -983,6 +993,13 @@ void LayerTreeHost::ApplyScrollAndScale(const ScrollAndScaleSet& info) {
   }
   if (!root_scroll_delta.IsZero() || info.page_scale_delta != 1.f)
     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) {