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

tools/clang/blink_gc_plugin/JsonWriter.h

Index: tools/clang/blink_gc_plugin/JsonWriter.h
diff --git a/tools/clang/blink_gc_plugin/JsonWriter.h b/tools/clang/blink_gc_plugin/JsonWriter.h
index 3fe79103bbadbadc21770b707584ba1648462ed4..45a3463e8c7a8ef73e1726679f7e8a2131bd1394 100644
--- a/tools/clang/blink_gc_plugin/JsonWriter.h
+++ b/tools/clang/blink_gc_plugin/JsonWriter.h
@@ -18,7 +18,7 @@ class JsonWriter {
     *os_ << "[";
     state_.push(false);
   }
-  void OpenList(const std::string key) {
+  void OpenList(const std::string& key) {
     Write(key);
     *os_ << ":";
     OpenList();
@@ -40,15 +40,15 @@ class JsonWriter {
     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 << "\"";
   }