Object.observe: Handle oldValue for elements with accessors properly.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 3dc6188997bdead58671bc9982202033e64f5e38..28729d013198e7b54e6763a5340a23d0d75e4554 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -1835,8 +1835,13 @@ class JSObject: public JSReceiver {
     return old_capacity + (old_capacity >> 1) + 16;
   }
 
+  PropertyType GetLocalPropertyType(String* name);
+  PropertyType GetLocalElementType(uint32_t index);
+  AccessorPair* GetLocalPropertyAccessorPair(String* name);
+  AccessorPair* GetLocalElementAccessorPair(uint32_t index);
+
   // Tells whether the index'th element is present and how it is stored.
-  enum LocalElementType {
+  enum LocalElementKind {
     // There is no element with given index.
     UNDEFINED_ELEMENT,
 
@@ -1853,7 +1858,7 @@ class JSObject: public JSReceiver {
     DICTIONARY_ELEMENT
   };
 
-  LocalElementType GetLocalElementType(uint32_t index);
+  LocalElementKind GetLocalElementKind(uint32_t index);
 
   MUST_USE_RESULT MaybeObject* SetFastElement(uint32_t index,
                                               Object* value,