[js weak refs] Add tests: WeakFactory keeps WeakCells alive

BUG=v8:8179

Change-Id: I43861e114b9f46847df9b02d0337709a685feb72
Reviewed-on: https://chromium-review.googlesource.com/c/1278810Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56771}

test/mjsunit/harmony/weakrefs/weak-factory-keeps-weak-cells-alive.js

+// Copyright 2018 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-weak-refs --expose-gc
+
+let cleanup_called = false;
+let cleanup = function(iter) {
+  assertFalse(cleanup_called);
+  let cells = [];
+  for (wc of iter) {
+    cells.push(wc);
+  }
+  assertEquals(cells.length, 1);
+  assertEquals(cells[0].holdings, "this is my cell");
+  cleanup_called = true;
+}
+
+let wf = new WeakFactory(cleanup);
+let o1 = {};
+let wc1 = wf.makeCell(o1, "this is my cell");
+
+gc();
+assertFalse(cleanup_called);
+
+// Drop the last references to o1.
+o1 = null;
+
+// Drop the last reference to the WeakCell. The WeakFactory keeps it alive, so
+// the cleanup function will be called as normal.
+wc1 = null;
+gc();
+assertFalse(cleanup_called);
+
+let timeout_func = function() {
+  assertTrue(cleanup_called);
+}
+
+setTimeout(timeout_func, 0);