Issue 5258 - Implement Filter::As<>() method to make working with filters easier from C++ code

compiled/filter/Filter.h

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present 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
@@ skipping 42 matching lines @@
   const String& BINDINGS_EXPORTED GetText() const
   {
     return mText;
   }
 
   OwnedString BINDINGS_EXPORTED Serialize() const;
 
   static Filter* BINDINGS_EXPORTED FromText(DependentString& text);
 
   template<typename T>
-  T* As();
+  T* As()
+  {
+    if ((mType & T::classType) != T::classType)
+      return nullptr;
+
+    return static_cast<T*>(this);
+  }
+
+  template<typename T>
+  const T* As() const
+  {
+    if ((mType & T::classType) != T::classType)
+      return nullptr;
+
+    return static_cast<const T*>(this);
+  }
 };
 
 typedef intrusive_ptr<Filter> FilterPtr;