Issue 5013 - Make parameter const ref when applicable.

src/JsError.h

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2017 eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
  */
 
 #ifndef ADBLOCK_PLUS_JS_ERROR_H
 #define ADBLOCK_PLUS_JS_ERROR_H
 
 #include <sstream>
 #include <stdexcept>
 #include <v8.h>
 
 namespace
 {
-  std::string ExceptionToString(const v8::Handle<v8::Value> exception,
-      const v8::Handle<v8::Message> message)
+  std::string ExceptionToString(const v8::Handle<v8::Value>& exception,
+      const v8::Handle<v8::Message>& message)
   {
     std::stringstream error;
     error << *v8::String::Utf8Value(exception);
     if (!message.IsEmpty())
     {
       error << " at ";
       error << *v8::String::Utf8Value(message->GetScriptResourceName());
       error << ":";
       error << message->GetLineNumber();
     }
     return error.str();
   }
 }
 
 namespace AdblockPlus
 {
   class JsError : public std::runtime_error
   {
   public:
-    JsError(const v8::Handle<v8::Value> exception,
-            const v8::Handle<v8::Message> message);
+    JsError(const v8::Handle<v8::Value>& exception,
+            const v8::Handle<v8::Message>& message);
   };
 }
 
 #endif