Oilpan: fix build after r353261.

third_party/WebKit/Source/core/html/HTMLAreaElement.h

Index: third_party/WebKit/Source/core/html/HTMLAreaElement.h
diff --git a/third_party/WebKit/Source/core/html/HTMLAreaElement.h b/third_party/WebKit/Source/core/html/HTMLAreaElement.h
index 1513f8f80ce413d59e63209f2cd93819ff8007d1..d9d361b49c01a131b08aa0591a029aaa5ec9600f 100644
--- a/third_party/WebKit/Source/core/html/HTMLAreaElement.h
+++ b/third_party/WebKit/Source/core/html/HTMLAreaElement.h
@@ -35,6 +35,7 @@ class Path;
 
 class CORE_EXPORT HTMLAreaElement final : public HTMLAnchorElement {
     DEFINE_WRAPPERTYPEINFO();
+    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(HTMLAreaElement);
 public:
     DECLARE_NODE_FACTORY(HTMLAreaElement);
 
@@ -48,6 +49,8 @@ public:
     // The parent map's image.
     HTMLImageElement* imageElement() const;
 
+    DEFINE_INLINE_VIRTUAL_TRACE() { HTMLAnchorElement::trace(visitor); }
+
 private:
     explicit HTMLAreaElement(Document&);
     ~HTMLAreaElement();