Add impl/comp scrolling instrumentation.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index c61e6bec0d0c99f7f90801f421dd448b928bb9cb..6506a2f00a574941e93ae6464222f08bec93f956 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -1804,11 +1804,21 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBegin(
   // Walk up the hierarchy and look for a scrollable layer.
   LayerImpl* potentially_scrolling_layer_impl = 0;
   for (; layer_impl; layer_impl = layer_impl->parent()) {
+    TRACE_EVENT_INSTANT1(
+        "impl-scroll", "LayerTreeHostImpl::ScrollBegin considering layer",
+        TRACE_EVENT_SCOPE_THREAD,
+        "layer_id", layer_impl->id());
+
     // The content layer can also block attempts to scroll outside the main
     // thread.
     ScrollStatus status = layer_impl->TryScroll(device_viewport_point, type);
     if (status == ScrollOnMainThread) {
       rendering_stats_instrumentation_->IncrementMainThreadScrolls();
+      TRACE_EVENT_INSTANT0(
+          "impl-scroll",
+          "LayerTreeHostImpl::ScrollBegin return ScrollOnMainThread",
+          TRACE_EVENT_SCOPE_THREAD);
+
       UMA_HISTOGRAM_BOOLEAN("TryScroll.SlowScroll", true);
       return ScrollOnMainThread;
     }
@@ -1817,17 +1827,35 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBegin(
     if (!scroll_layer_impl)
       continue;
 
+    TRACE_EVENT_INSTANT1(
+        "impl-scroll",
+        "LayerTreeHostImpl::ScrollBegin considering scroll layer",
+        TRACE_EVENT_SCOPE_THREAD,
+        "layer_id", scroll_layer_impl->id());
+
     status = scroll_layer_impl->TryScroll(device_viewport_point, type);
 
     // If any layer wants to divert the scroll event to the main thread, abort.
     if (status == ScrollOnMainThread) {
       rendering_stats_instrumentation_->IncrementMainThreadScrolls();
+      TRACE_EVENT_INSTANT0(
+          "impl-scroll",
+          "LayerTreeHostImpl::ScrollBegin return ScrollOnMainThread",
+          TRACE_EVENT_SCOPE_THREAD);
+
       UMA_HISTOGRAM_BOOLEAN("TryScroll.SlowScroll", true);
       return ScrollOnMainThread;
     }
 
-    if (status == ScrollStarted && !potentially_scrolling_layer_impl)
+    if (status == ScrollStarted && !potentially_scrolling_layer_impl) {
       potentially_scrolling_layer_impl = scroll_layer_impl;
+      TRACE_EVENT_INSTANT1(
+          "impl-scroll",
+          "LayerTreeHostImpl::ScrollBegin scroll layer is the potentially "
+          "scrolling layer",
+          TRACE_EVENT_SCOPE_THREAD,
+          "layer_id", scroll_layer_impl->id());
+    }
   }
 
   // When hiding top controls is enabled and the controls are hidden or
@@ -1846,6 +1874,10 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBegin(
     wheel_scrolling_ = (type == Wheel);
     rendering_stats_instrumentation_->IncrementImplThreadScrolls();
     client_->RenewTreePriority();
+    TRACE_EVENT_ASYNC_BEGIN0(
+        "impl-scroll",
+        "LayerTreeHostImpl::ScrollBegin",
+        this);
     UMA_HISTOGRAM_BOOLEAN("TryScroll.SlowScroll", false);
     return ScrollStarted;
   }
@@ -2110,6 +2142,10 @@ void LayerTreeHostImpl::ClearCurrentlyScrollingLayer() {
 void LayerTreeHostImpl::ScrollEnd() {
   if (top_controls_manager_)
     top_controls_manager_->ScrollEnd();
+  TRACE_EVENT_ASYNC_END0(
+      "impl-scroll",
+      "LayerTreeHostImpl::ScrollBegin",
+      this);
   ClearCurrentlyScrollingLayer();
   StartScrollbarAnimation();
 }
@@ -2195,6 +2231,14 @@ scoped_ptr<ScrollAndScaleSet> LayerTreeHostImpl::ProcessScrollDeltas() {
   scoped_ptr<ScrollAndScaleSet> scroll_info(new ScrollAndScaleSet());
 
   CollectScrollDeltas(scroll_info.get(), active_tree_->root_layer());
+
+  if (!scroll_info->scrolls.empty()) {
+    TRACE_EVENT_INSTANT0(
+        "impl-scroll",
+        "LayerTreeHostImpl::ProcessScrollDeltas found and set scroll deltas",
+        TRACE_EVENT_SCOPE_THREAD);
+  }
+
   scroll_info->page_scale_delta = active_tree_->page_scale_delta();
   active_tree_->set_sent_page_scale_delta(scroll_info->page_scale_delta);