Removed superfluous cast.

R=jkummerow@chromium.org

Review URL: https://chromiumcodereview.appspot.com/10696119

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11992 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/objects-inl.h

@@ -4794,7 +4794,7 @@ bool String::AsArrayIndex(uint32_t* index) {
 
 
 Object* JSReceiver::GetPrototype() {
-  return HeapObject::cast(this)->map()->prototype();
+  return map()->prototype();
 }