Issue 5137 - [emscripten] Added basic filter storage implementation

compiled/subscription/DownloadableSubscription.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
@@ skipping 35 matching lines @@
       GetErrorCount, SetErrorCount);
   SUBSCRIPTION_PROPERTY(uint64_t, mDataRevision, NONE,
       GetDataRevision, SetDataRevision);
   SUBSCRIPTION_STRING_PROPERTY(mRequiredVersion, NONE,
       GetRequiredVersion, SetRequiredVersion);
   SUBSCRIPTION_PROPERTY(int, mDownloadCount, NONE,
       GetDownloadCount, SetDownloadCount);
 
   EMSCRIPTEN_KEEPALIVE OwnedString Serialize() const;
 };
+
+template<>
+inline DownloadableSubscription* Subscription::As<DownloadableSubscription>()
+{
+  if (mType != Type::DOWNLOADABLE)
+    return nullptr;
+
+  return static_cast<DownloadableSubscription*>(this);
+}