Trace main thread commit in flow events.

cc/output/latency_info_swap_promise.cc

Index: cc/output/latency_info_swap_promise.cc
diff --git a/cc/output/latency_info_swap_promise.cc b/cc/output/latency_info_swap_promise.cc
index afc0f2d85023f45ce6c4945c04a856af0cd669fb..5e08470fb07bcf489f9a0074befdc27a6529958a 100644
--- a/cc/output/latency_info_swap_promise.cc
+++ b/cc/output/latency_info_swap_promise.cc
@@ -5,6 +5,7 @@
 #include "cc/output/latency_info_swap_promise.h"
 
 #include "base/logging.h"
+#include "base/trace_event/trace_event.h"
 
 namespace {
 ui::LatencyComponentType DidNotSwapReasonToLatencyComponentType(
@@ -49,4 +50,11 @@ int64 LatencyInfoSwapPromise::TraceId() const {
   return latency_.trace_id;
 }
 
+// Trace the original LatencyInfo of a LatencyInfoSwapPromise
+void LatencyInfoSwapPromise::OnCommit() {
+  TRACE_EVENT_FLOW_STEP0("input,benchmark", "LatencyInfo.Flow",
+                         TRACE_ID_DONT_MANGLE(TraceId()),
+                         "HanldeInputEventMainCommit");
+}
+
 }  // namespace cc