Oilpan: fix build after r180052.

Source/core/dom/FullscreenElementStack.h

Index: Source/core/dom/FullscreenElementStack.h
diff --git a/Source/core/dom/FullscreenElementStack.h b/Source/core/dom/FullscreenElementStack.h
index 41dd652cefb6f5996cfd9ae3c5031e131d77418d..6a77a3ad3b21c6f696dc8dcd2ee6be3eea138f2d 100644
--- a/Source/core/dom/FullscreenElementStack.h
+++ b/Source/core/dom/FullscreenElementStack.h
@@ -36,6 +36,7 @@
 #include "platform/geometry/LayoutRect.h"
 #include "wtf/Deque.h"
 #include "wtf/RefPtr.h"
+#include "wtf/TypeTraits.h"
 #include "wtf/Vector.h"
 
 namespace blink {
@@ -140,4 +141,9 @@ inline FullscreenElementStack* FullscreenElementStack::fromIfExists(Document& do
 
 } // namespace blink
 
+// Needed by the HeapVector<> element stack.
+template<>struct WTF::IsPod<blink::FullscreenElementStack::RequestType> {
+    static const bool value = true;
+};
+
 #endif // FullscreenElementStack_h