Implement a new flow event API that allows binding flow events and regular events.

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 ebfe7d2eeb20d8bd5988cde1ea75816bf99d4706..cb4170c9f81c08c5224fd5e6ec0a08dd3b7e57f2 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -804,6 +804,9 @@ void LayerTreeHost::ApplyScrollAndScale(ScrollAndScaleSet* info) {
   ScopedPtrVector<SwapPromise>::iterator it = info->swap_promises.begin();
   for (; it != info->swap_promises.end(); ++it) {
     scoped_ptr<SwapPromise> swap_promise(info->swap_promises.take(it));
+    TRACE_EVENT_WITH_FLOW0("toplevel", "LayerTreeHost::ApplyScrollAndScale",
+                           TRACE_ID_DONT_MANGLE(swap_promise->TraceId()),
+                           TRACE_EVENT_FLAG_FLOW_IN | TRACE_EVENT_FLAG_FLOW_OUT);
     TRACE_EVENT_FLOW_STEP0("input",
                            "LatencyInfo.Flow",
                            TRACE_ID_DONT_MANGLE(swap_promise->TraceId()),