Make WTF::IsGarbageCollectedType<> work for GC mixin instances.

third_party/WebKit/Source/core/layout/LayoutBlock.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2007 David Smith (catfish.man@gmail.com)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 331 matching lines @@
     // is called the same number of times.
     // finishDelayUpdateScrollInfo returns true when it marked something for layout.
     // It will also return a map of saved scroll positions that the caller should restore
     // on the given scrollable areas after performing the layout.
     // This can be necessary because Flexbox's multi-pass layout can lose the scroll position.
     // TODO(cbiesinger): This is a temporary hack. The right solution is to delay the scroll
     // clamping that currently happens in PaintLayerScrollableArea::updateAfterLayout to only
     // happen after all layout is done, i.e. during updateLayerPositionsAfterLayout. However,
     // that currently fails a layout test. To fix this bug in time for M50, we use this temporary
     // hack. The real fix is tracked in crbug.com/600036
-    typedef WTF::HashMap<PaintLayerScrollableArea*, DoublePoint> ScrollPositionMap;
+    typedef PersistentHeapHashMap<Member<PaintLayerScrollableArea>, DoublePoint> ScrollPositionMap;
     static void startDelayUpdateScrollInfo();
     static bool finishDelayUpdateScrollInfo(SubtreeLayoutScope*, ScrollPositionMap*);
 
     void updateScrollInfoAfterLayout();
 
     void styleWillChange(StyleDifference, const ComputedStyle& newStyle) override;
     void styleDidChange(StyleDifference, const ComputedStyle* oldStyle) override;
     void updateFromStyle() override;
 
     // Returns true if non-visible overflow should be respected. Otherwise hasOverflowClip() will be
@@ skipping 156 matching lines @@
     // FIXME: This is temporary as we move code that accesses block flow
     // member variables out of LayoutBlock and into LayoutBlockFlow.
     friend class LayoutBlockFlow;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutBlock, isLayoutBlock());
 
 } // namespace blink
 
 #endif // LayoutBlock_h