Implement scroll handler latency tracking

cc/base/latency_info_swap_promise.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/base/latency_info_swap_promise.h"
 
 #include "base/logging.h"
 
 namespace {
   ui::LatencyComponentType DidNotSwapReasonToLatencyComponentType(
@@ skipping 27 matching lines @@
 }
 
 void LatencyInfoSwapPromise::DidNotSwap(DidNotSwapReason reason) {
   latency_.AddLatencyNumber(DidNotSwapReasonToLatencyComponentType(reason),
                             0, 0);
   // TODO(miletus): Turn this back on once per-event LatencyInfo tracking
   // is enabled in GPU side.
   // DCHECK(latency_.terminated);
 }
 
+int64 LatencyInfoSwapPromise::TraceId() const {
+  return latency_.trace_id;
+}
+
 }  // namespace cc