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

cc/scheduler/begin_frame_tracker.cc

Index: cc/scheduler/begin_frame_tracker.cc
diff --git a/cc/scheduler/begin_frame_tracker.cc b/cc/scheduler/begin_frame_tracker.cc
index 3b526ccad0002b71088d8848817783fb98787429..9dad2dd16b3d38b1933bbbbbd59e301fdaa20df9 100644
--- a/cc/scheduler/begin_frame_tracker.cc
+++ b/cc/scheduler/begin_frame_tracker.cc
@@ -19,6 +19,9 @@ BeginFrameTracker::~BeginFrameTracker() {
 
 void BeginFrameTracker::Start(BeginFrameArgs new_args) {
   // Trace the frame time being passed between BeginFrameTrackers.
+  TRACE_EVENT_WITH_FLOW0("toplevel", "BeginFrameTracker::Start",
+                         new_args.frame_time.ToInternalValue(),
+                         TRACE_EVENT_FLAG_FLOW_IN | TRACE_EVENT_FLAG_FLOW_OUT);
   TRACE_EVENT_FLOW_STEP0(
       TRACE_DISABLED_BY_DEFAULT("cc.debug.scheduler.frames"), "BeginFrameArgs",
       new_args.frame_time.ToInternalValue(), location_string_);