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

components/tracing/trace_config_file.cc

 // 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.
 
 #include "components/tracing/trace_config_file.h"
 
 #include <string>
 
 #include "base/command_line.h"
 #include "base/files/file_util.h"
@@ skipping 69 matching lines @@
                               &trace_config_file_content,
                               kTraceConfigFileSizeLimit)) {
     return;
   }
   is_enabled_ = ParseTraceConfigFileContent(trace_config_file_content);
 }
 
 TraceConfigFile::~TraceConfigFile() {
 }
 
-bool TraceConfigFile::ParseTraceConfigFileContent(std::string content) {
+bool TraceConfigFile::ParseTraceConfigFileContent(const std::string& content) {
   scoped_ptr<base::Value> value(base::JSONReader::Read(content));
   if (!value || !value->IsType(base::Value::TYPE_DICTIONARY))
     return false;
 
   scoped_ptr<base::DictionaryValue> dict(
       static_cast<base::DictionaryValue*>(value.release()));
 
   base::DictionaryValue* trace_config_dict = NULL;
   if (!dict->GetDictionary(kTraceConfigParam, &trace_config_dict))
     return false;
@@ skipping 30 matching lines @@
 }
 
 #if !defined(OS_ANDROID)
 base::FilePath TraceConfigFile::GetResultFile() const {
   DCHECK(IsEnabled());
   return result_file_;
 }
 #endif
 
 }  // namespace tracing