[oilpan] Wrap NodeVector into CollectionRoot for ensure that contained nodes stay alive.

Source/heap/Handle.h

Index: Source/heap/Handle.h
diff --git a/Source/heap/Handle.h b/Source/heap/Handle.h
index d8d0f5fae4377f30cb45e0a54212a96ad4104855..602270a60cfbdd78f352a2dec0d1db07d49e0b8c 100644
--- a/Source/heap/Handle.h
+++ b/Source/heap/Handle.h
@@ -1128,10 +1128,25 @@ struct CollectionVisitingTrait<WTF::HashMap<K, V> > {
     }
 };
 
+template<typename Collection> class CollectionRoot;
+
+// Used to inject correctly typed operator[] into CollectionRoot when we are wrapping Vector.
+template<typename T> class IndexingBehavior { };
+
+template<typename T, size_t inlineCapacity, typename Allocator>
+class IndexingBehavior<CollectionRoot<Vector<T, inlineCapacity, Allocator> > > {
+    typedef CollectionRoot<Vector<T, inlineCapacity, Allocator> > CollectionRootType;
+public:
+    T& operator[] (size_t i) { return (**static_cast<CollectionRootType*>(this))[i]; }
+    const T& operator[] (size_t i) const { return (**static_cast<const CollectionRootType*>(this))[i]; }
+};
+
 template<typename Collection>
-class CollectionRoot : public PersistentBase {
+class CollectionRoot : public PersistentBase, public IndexingBehavior<CollectionRoot<Collection> > {
 public:
     typedef Collection CollectionType;
+    typedef typename Collection::iterator iterator;
+    typedef typename Collection::const_iterator const_iterator;
 
     CollectionRoot() { }
     explicit CollectionRoot(size_t size) : m_collection(Collection(size)) { }