Cleanup: Pass std::string as const reference from tools/clang/

tools/clang/blink_gc_plugin/JsonWriter.h

 // Copyright 2014 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 TOOLS_BLINK_GC_PLUGIN_JSON_WRITER_H_
 #define TOOLS_BLINK_GC_PLUGIN_JSON_WRITER_H_
 
 #include "llvm/Support/raw_ostream.h"
 
 // Helper to write information for the points-to graph.
 class JsonWriter {
  public:
   static JsonWriter* from(std::unique_ptr<llvm::raw_ostream> os) {
     return os ? new JsonWriter(std::move(os)) : 0;
   }
   void OpenList() {
     Separator();
     *os_ << "[";
     state_.push(false);
   }
-  void OpenList(const std::string key) {
+  void OpenList(const std::string& key) {
     Write(key);
     *os_ << ":";
     OpenList();
   }
   void CloseList() {
     *os_ << "]";
     state_.pop();
   }
   void OpenObject() {
     Separator();
     *os_ << "{";
     state_.push(false);
   }
   void CloseObject() {
     *os_ << "}\n";
     state_.pop();
   }
   void Write(const size_t val) {
     Separator();
     *os_ << val;
   }
-  void Write(const std::string val) {
+  void Write(const std::string& val) {
     Separator();
     *os_ << "\"" << val << "\"";
   }
-  void Write(const std::string key, const size_t val) {
+  void Write(const std::string& key, const size_t val) {
     Separator();
     *os_ << "\"" << key << "\":" << val;
   }
-  void Write(const std::string key, const std::string val) {
+  void Write(const std::string& key, const std::string& val) {
     Separator();
     *os_ << "\"" << key << "\":\"" << val << "\"";
   }
  private:
   JsonWriter(std::unique_ptr<llvm::raw_ostream> os) : os_(std::move(os)) {}
   void Separator() {
     if (state_.empty())
       return;
     if (state_.top()) {
       *os_ << ",";
       return;
     }
     state_.top() = true;
   }
   std::unique_ptr<llvm::raw_ostream> os_;
   std::stack<bool> state_;
 };
 
 #endif // TOOLS_BLINK_GC_PLUGIN_JSON_WRITER_H_