Add a pure virtual method to the GarbageCollectedMixin class

Source/core/html/imports/HTMLImportLoader.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 37 matching lines @@
 class HTMLImportsController;
 
 
 //
 // Owning imported Document lifetime. It also implements ResourceClient through ResourceOwner
 // to feed fetched bytes to the DocumentWriter of the imported document.
 // HTMLImportLoader is owned by HTMLImportsController.
 //
 //
 class HTMLImportLoader final : public NoBaseWillBeGarbageCollectedFinalized<HTMLImportLoader>, public ResourceOwner<RawResource>, public DocumentParserClient {
+    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(HTMLImportLoader);
 public:
     enum State {
         StateLoading,
         StateWritten,
         StateParsed,
         StateLoaded,
         StateError
     };
 
     static PassOwnPtrWillBeRawPtr<HTMLImportLoader> create(HTMLImportsController* controller)
@@ skipping 59 matching lines @@
     WillBeHeapVector<RawPtrWillBeMember<HTMLImportChild> > m_imports;
     State m_state;
     RefPtrWillBeMember<Document> m_document;
     RefPtrWillBeMember<DocumentWriter> m_writer;
     RefPtrWillBeMember<CustomElementSyncMicrotaskQueue> m_microtaskQueue;
 };
 
 } // namespace blink
 
 #endif // HTMLImportLoader_h