[turbofan] Assign proper type to Object.prototype.toString().

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index aed708b90f4c76dda51b3645074c39f915db243b..8ddeb2a520930a9000329a554477bc367e901356 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -6996,6 +6996,7 @@ class Script: public Struct {
   V(Object, assign, ObjectAssign)                           \
   V(Object, create, ObjectCreate)                           \
   V(Object.prototype, hasOwnProperty, ObjectHasOwnProperty) \
+  V(Object.prototype, toString, ObjectToString)             \
   V(RegExp.prototype, compile, RegExpCompile)               \
   V(RegExp.prototype, exec, RegExpExec)                     \
   V(RegExp.prototype, test, RegExpTest)                     \