Handle setters on the prototype chain efficiently by default.

Note that changes on the prototype chain should already be handled by
StubCompiler::CheckPrototypes, if not, it is a bug there and not in
LookupForWrite. Furthermore, neither test262 nor WebKit layout tests hint at a
compability issue, so it should be safe to remove a redundant check.

The good point about this CL: It improves Box2D's performance by 22%.

Review URL: https://chromiumcodereview.appspot.com/10695197

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12079 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/ic.cc

@@ -1316,11 +1316,9 @@ static bool LookupForWrite(Handle<JSObject> receiver,
                            LookupResult* lookup) {
   receiver->LocalLookup(*name, lookup);
   if (!StoreICableLookup(lookup)) {
-    // 2nd chance: There can be accessors somewhere in the prototype chain, but
-    // for compatibility reasons we have to hide this behind a flag. Note that
-    // we explicitly exclude native accessors for now, because the stubs are not
-    // yet prepared for this scenario.
-    if (!FLAG_es5_readonly) return false;
+    // 2nd chance: There can be accessors somewhere in the prototype chain. Note
+    // that we explicitly exclude native accessors for now, because the stubs
+    // are not yet prepared for this scenario.
     receiver->Lookup(*name, lookup);
     if (!lookup->IsCallbacks()) return false;
     Handle<Object> callback(lookup->GetCallbackObject());