Fix async/await memory leak

test/mjsunit/harmony/async-await-loop.js

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --harmony-async-await --allow-natives-syntax --max-old-space-size 10
+
+let out;
+
+async function longLoop() {
+  for (let i = 0; i < 100000; i++) await undefined;
+  out = 1;
+}
+
+longLoop();
+
+%RunMicrotasks();
+
+assertEquals(1, out);
+
+async function thrower() { throw undefined; }
+
+async function throwLoop() {
+  for (let i = 0; i < 100000; i++) {
+    try { await thrower(); }
+    catch (e) {}
+  }
+  out = 2;
+}
+
+throwLoop();
+
+%RunMicrotasks();
+
+assertEquals(2, out);