[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 {
+
+const char TracingAgent::kChromeTracingAgentName[] = "chrome";
+const char TracingAgent::kETWTracingAgentName[] = "etw";
+const char TracingAgent::kCrOSTracingAgentName[] = "cros";
+const char TracingAgent::kPowerTracingAgentName[] = "battor";
+
+}  // namespace trace_event
+}  // namespace base