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

components/tracing/trace_config_file.cc

Index: components/tracing/trace_config_file.cc
diff --git a/components/tracing/trace_config_file.cc b/components/tracing/trace_config_file.cc
index 796eb1b7cb5dae5c923ddef43b1589cdd26eb243..769d527ca8bd5dbc2217c9ba91444b8db80aea16 100644
--- a/components/tracing/trace_config_file.cc
+++ b/components/tracing/trace_config_file.cc
@@ -87,7 +87,7 @@ TraceConfigFile::TraceConfigFile()
 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;