[Startup Tracing] Enable startup tracing from config file

base/trace_event/startup_tracing.h

Index: base/trace_event/startup_tracing.h
diff --git a/base/trace_event/startup_tracing.h b/base/trace_event/startup_tracing.h
new file mode 100644
index 0000000000000000000000000000000000000000..b237d14962217e6b4a0f5dd921bac1c1977efbb1
--- /dev/null
+++ b/base/trace_event/startup_tracing.h
@@ -0,0 +1,22 @@
+// Copyright (c) 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.
+
+#ifndef BASE_TRACE_EVENT_STARTUP_TRACING_H_
+#define BASE_TRACE_EVENT_STARTUP_TRACING_H_
+
+#include "base/base_export.h"
+
+namespace base {
+namespace trace_event {
+
+// Enable startup tracing according to the trace config file. If the trace
+// config file does not exist, it will do nothing. This is designed to be used
+// by Telemetry. Telemetry will stop tracing via DevTools later. To avoid
+// conflict, this should not be used when --trace-startup is enabled.
+BASE_EXPORT void EnableStartupTracingIfConfigFileExists();
+
+}  // namespace trace_event
+}  // namespace base
+
+#endif  // BASE_TRACE_EVENT_STARTUP_TRACING_H_