Refactor prototype setting code and expose SetPrototype to public V8 API.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 1ed8e4c8fc57c5146a038035b6bdfd0927633460..3d60d306eb48c39c0c17b115b76e723f91e5ad62 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -1317,6 +1317,9 @@ class JSObject: public HeapObject {
   // Return the object's prototype (might be Heap::null_value()).
   inline Object* GetPrototype();
 
+  // Set the object's prototype (only JSObject and null are allowed).
+  Object* SetPrototype(Object* value, bool skip_hidden_prototypes);
+
   // Tells whether the index'th element is present.
   inline bool HasElement(uint32_t index);
   bool HasElementWithReceiver(JSObject* receiver, uint32_t index);