Issue 5013 - Make parameter const ref when applicable.

src/JsError.cpp

 /*
  * 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/>.
  */
 
 #include "JsError.h"
 
-AdblockPlus::JsError::JsError(const v8::Handle<v8::Value> exception,
-    const v8::Handle<v8::Message> message)
+AdblockPlus::JsError::JsError(const v8::Handle<v8::Value>& exception,
+    const v8::Handle<v8::Message>& message)
   : std::runtime_error(ExceptionToString(exception, message))
 {
 }