Oilpan: fix build after r197063, move HitTestCache to the heap.

Source/core/layout/HitTestResult.h

Index: Source/core/layout/HitTestResult.h
diff --git a/Source/core/layout/HitTestResult.h b/Source/core/layout/HitTestResult.h
index dea3ffbedf7882a27a437861d505c28e3182bfcf..9deba8970168a17b170bae7776365e48c37188fc 100644
--- a/Source/core/layout/HitTestResult.h
+++ b/Source/core/layout/HitTestResult.h
@@ -34,6 +34,7 @@
 #include "wtf/ListHashSet.h"
 #include "wtf/OwnPtr.h"
 #include "wtf/RefPtr.h"
+#include "wtf/VectorTraits.h"
 
 namespace blink {
 
@@ -172,4 +173,16 @@ private:
 
 } // namespace blink
 
+#if ENABLE(OILPAN)
+// TODO(sof): the trait override/specialization is needed by HitTestCache's
+// HeapVector<> to handle unused slots. It is not correct for HitTestResult
+// in the wider sense of what canInitializeWithMemset provides, so fix this
+// by introducing a trait that encompasses "unused slot" handling and use it
+// here instead.
+//
+// Until that time, make this trait specialization conditional on OILPAN to
+// limit exposure.
+WTF_ALLOW_INIT_WITH_MEM_FUNCTIONS(blink::HitTestResult);
+#endif
+
 #endif // HitTestResult_h