Trace main thread commit in flow events.

cc/trees/layer_tree_host.h

Index: cc/trees/layer_tree_host.h
diff --git a/cc/trees/layer_tree_host.h b/cc/trees/layer_tree_host.h
index 449d8e4e09a0f614e44ead23cf8677e65fa2dedb..a159e31d1acf6293568b5d61c880e8f670ac3afe 100644
--- a/cc/trees/layer_tree_host.h
+++ b/cc/trees/layer_tree_host.h
@@ -18,6 +18,7 @@
 #include "base/memory/weak_ptr.h"
 #include "base/time/time.h"
 #include "base/timer/timer.h"
+#include "base/trace_event/trace_event.h"
 #include "cc/animation/animation_events.h"
 #include "cc/base/cc_export.h"
 #include "cc/base/scoped_ptr_vector.h"
@@ -311,6 +312,16 @@ class CC_EXPORT LayerTreeHost {
 
   size_t num_queued_swap_promises() const { return swap_promise_list_.size(); }
 
+  void trace_commit() {
+    for (unsigned int i = 0; i < num_queued_swap_promises(); i++) {
+      SwapPromise* swap_promise_ = swap_promise_list_.at(i);
+
+      TRACE_EVENT_FLOW_STEP0("input,benchmark", "LatencyInfo.Flow",
+                             TRACE_ID_DONT_MANGLE(swap_promise_->TraceId()),

[Yufeng Shen (Slow to review), 2015/06/15 23:48:25]

Not all SwapPromise is carrying LatencyInfo with it, so its TraceId()
can'be always interpreted as the trace_id for LatencyInfo.

I think you should have a
virtual SwapPromise::OnCommit() {}

and only in LatencyInfoSwapPromise::OnCommit()
you can use the trace_id for adding the flow event.

+                             "HanldeInputEventMainCommit");
+    }
+  }
+
   void set_surface_id_namespace(uint32_t id_namespace);
   SurfaceSequence CreateSurfaceSequence();