Enable startup tracing from command line flags in components/tracing

components/tracing/common/trace_startup.h

+// Copyright 2017 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 COMPONENTS_TRACING_COMMON_TRACE_STARTUP_H
+#define COMPONENTS_TRACING_COMMON_TRACE_STARTUP_H
+
+#include "components/tracing/tracing_export.h"
+
+namespace tracing {
+
+// Enables TraceLog with config based on the command line flags of the process.
+// If |can_access_file_system| is false then TraceLog is not enabled in case it
+// is required to read config file to start tracing.
+void TRACING_EXPORT EnableStartupTracingIfNeeded(bool can_access_file_system);
+
+}  // namespace tracing
+
+#endif  // COMPONENTS_TRACING_COMMON_TRACE_STARTUP_H