[promises] Set promise context's closure to be an empty function

This is similar to how the native context has an empty function set up as it's closure field.

BUG=666984

Review-Url: https://codereview.chromium.org/2528603002
Cr-Commit-Position: refs/heads/master@{#41212}

src/promise-utils.cc

@@ -44,6 +44,8 @@ void PromiseUtils::CreateResolvingFunctions(Isolate* isolate,
       isolate->factory()->NewPromiseResolvingFunctionContext(
           kPromiseContextLength);
   context->set_native_context(*isolate->native_context());
+  // We set the closure to be an empty function, same as native context.
+  context->set_closure(isolate->native_context()->closure());
   context->set(kAlreadyVisitedSlot, Smi::kZero);
   context->set(kPromiseSlot, *promise);
   context->set(kDebugEventSlot, *debug_event);