Issue 5137 - [emscripten] Added basic filter storage implementation

compiled/subscription/UserDefinedSubscription.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 12 matching lines @@
 
 class UserDefinedSubscription : public Subscription
 {
 private:
   int mDefaults;
 
 public:
   explicit UserDefinedSubscription(const String& id);
   BINDINGS_EXPORTED bool IsDefaultFor(const Filter* filter) const;
   BINDINGS_EXPORTED void MakeDefaultFor(const Filter* filter);
+  BINDINGS_EXPORTED bool IsGeneric() const
+  {
+    return mDefaults == 0;
+  }
   BINDINGS_EXPORTED void InsertFilterAt(Filter* filter, unsigned pos);
   BINDINGS_EXPORTED bool RemoveFilterAt(unsigned pos);
   BINDINGS_EXPORTED OwnedString Serialize() const;
 };
+
+template<>
+inline UserDefinedSubscription* Subscription::As<UserDefinedSubscription>()
+{
+  if (mType != Type::USERDEFINED)
+    return nullptr;
+
+  return static_cast<UserDefinedSubscription*>(this);
+}