Cleanup: Pass std::string as const reference if possible

components/tracing/trace_config_file.h

 // 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 COMPONENTS_TRACING_TRACE_CONFIG_FILE_H_
 #define COMPONENTS_TRACING_TRACE_CONFIG_FILE_H_
 
 #include "base/files/file_path.h"
 #include "base/macros.h"
 #include "base/trace_event/trace_config.h"
@@ skipping 63 matching lines @@
   base::FilePath GetResultFile() const;
 #endif
 
  private:
   // This allows constructor and destructor to be private and usable only
   // by the Singleton class.
   friend struct base::DefaultSingletonTraits<TraceConfigFile>;
   TraceConfigFile();
   ~TraceConfigFile();
 
-  bool ParseTraceConfigFileContent(std::string content);
+  bool ParseTraceConfigFileContent(const std::string& content);
 
   bool is_enabled_;
   base::trace_event::TraceConfig trace_config_;
   int startup_duration_;
   base::FilePath result_file_;
 
   DISALLOW_COPY_AND_ASSIGN(TraceConfigFile);
 };
 
 }  // namespace tracing
 
 #endif  // COMPONENTS_TRACING_TRACE_CONFIG_FILE_H_