[Tracing Clock Sync] Add TracingAgent interface in Chrome

base/trace_event/tracing_agent.cc

+// Copyright 2015 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 "base/trace_event/tracing_agent.h"
+
+namespace base {
+namespace trace_event {
+
+TracingAgent::~TracingAgent() {}
+
+bool TracingAgent::SupportsExplicitClockSync() {
+  return false;
+}
+
+void TracingAgent::RecordClockSyncMarker(
+    int sync_id,
+    const RecordClockSyncMarkerCallback& callback) {
+  DCHECK(SupportsExplicitClockSync());
+}
+
+
+}  // namespace trace_event
+}  // namespace base