[runtime] Speed up Object::ToObject with JSReceiver argument.

This avoids having to read the context and call through from the inlined
path in the JSReceiver case.

BUG=

Review URL: https://codereview.chromium.org/1698463002

Cr-Commit-Position: refs/heads/master@{#33949}

src/objects-inl.h

@@ -1143,8 +1143,8 @@ bool Object::FitsRepresentation(Representation representation) {
 // static
 MaybeHandle<JSReceiver> Object::ToObject(Isolate* isolate,
                                          Handle<Object> object) {
-  return ToObject(
-      isolate, object, handle(isolate->context()->native_context(), isolate));
+  if (object->IsJSReceiver()) return Handle<JSReceiver>::cast(object);
+  return ToObject(isolate, object, isolate->native_context());
 }