Issue 5013 - Make parameter const ref when applicable.

include/AdblockPlus/FilterEngine.h

Index: include/AdblockPlus/FilterEngine.h
===================================================================
--- a/include/AdblockPlus/FilterEngine.h
+++ b/include/AdblockPlus/FilterEngine.h
@@ -201,17 +201,17 @@
 
     /**
      * Callback type invoked when the filters change.
      * The first parameter is the action event code (see
      * [FilterNotifier.triggerListeners](https://adblockplus.org/jsdoc/adblockpluscore/FilterNotifier.html#.triggerListeners)
      * for the full list).
      * The second parameter is the filter/subscription object affected, if any.
      */
-    typedef std::function<void(const std::string&, const JsValuePtr)> FilterChangeCallback;
+    typedef std::function<void(const std::string&, const JsValuePtr&)> FilterChangeCallback;
 
     /**
      * Container of name-value pairs representing a set of preferences.
      */
     typedef std::map<std::string, AdblockPlus::JsValuePtr> Prefs;
 
     /**
      * Callback type invoked when a new notification should be shown.
@@ -430,30 +430,30 @@
      */
     JsValuePtr GetPref(const std::string& pref) const;
 
     /**
      * Sets a preference value.
      * @param pref Preference name.
      * @param value New value of the preference.
      */
-    void SetPref(const std::string& pref, JsValuePtr value);
+    void SetPref(const std::string& pref, const JsValuePtr& value);
 
     /**
      * Extracts the host from a URL.
      * @param url URL to extract the host from.
      * @return Extracted host.
      */
     std::string GetHostFromURL(const std::string& url) const;
 
     /**
      * Sets the callback invoked when an application update becomes available.
      * @param callback Callback to invoke.
      */
-    void SetUpdateAvailableCallback(UpdateAvailableCallback callback);
+    void SetUpdateAvailableCallback(const UpdateAvailableCallback& callback);
 
     /**
      * Removes the callback invoked when an application update becomes
      * available.
      */
     void RemoveUpdateAvailableCallback();
 
     /**
@@ -469,17 +469,17 @@
      *        `FilterEngine::SetUpdateAvailableCallback()`.
      */
     void ForceUpdateCheck(const UpdateCheckDoneCallback& callback = UpdateCheckDoneCallback());
 
     /**
      * Sets the callback invoked when the filters change.
      * @param callback Callback to invoke.
      */
-    void SetFilterChangeCallback(FilterChangeCallback callback);
+    void SetFilterChangeCallback(const FilterChangeCallback& callback);
 
     /**
      * Removes the callback invoked when the filters change.
      */
     void RemoveFilterChangeCallback();
 
     /**
      * Stores the value indicating what connection types are allowed, it is
@@ -529,20 +529,20 @@
     int updateCheckId;
     static const std::map<ContentType, std::string> contentTypes;
 
     explicit FilterEngine(const JsEnginePtr& jsEngine);
 
     FilterPtr CheckFilterMatch(const std::string& url,
                                ContentTypeMask contentTypeMask,
                                const std::string& documentUrl) const;
-    void UpdateAvailable(UpdateAvailableCallback callback, JsValueList& params) const;
+    void UpdateAvailable(const UpdateAvailableCallback& callback, const JsValueList& params) const;
     void UpdateCheckDone(const std::string& eventName,
-                         UpdateCheckDoneCallback callback, JsValueList& params);
-    void FilterChanged(FilterChangeCallback callback, JsValueList& params) const;
+                         const UpdateCheckDoneCallback& callback, const JsValueList& params);
+    void FilterChanged(const FilterChangeCallback& callback, const JsValueList& params) const;
     void ShowNotification(const ShowNotificationCallback& callback,
       const JsValueList& params) const;
     FilterPtr GetWhitelistingFilter(const std::string& url,
       ContentTypeMask contentTypeMask, const std::string& documentUrl) const;
     FilterPtr GetWhitelistingFilter(const std::string& url,
       ContentTypeMask contentTypeMask,
       const std::vector<std::string>& documentUrls) const;
   };