Add a basic demo mode browser test.

chrome/browser/chromeos/login/test/app_window_waiter.cc

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "chrome/browser/chromeos/login/test/app_window_waiter.h"
+
+#include "apps/app_window.h"
+#include "content/public/test/test_utils.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace chromeos {
+
+AppWindowWaiter::AppWindowWaiter(apps::AppWindowRegistry* registry,
+                                 const std::string& app_id)
+    : registry_(registry), app_id_(app_id), window_(NULL), running_(false) {
+  registry_->AddObserver(this);
+}
+
+AppWindowWaiter::~AppWindowWaiter() {
+  registry_->RemoveObserver(this);
+}
+
+apps::AppWindow* AppWindowWaiter::Wait() {
+  window_ = registry_->GetCurrentAppWindowForApp(app_id_);
+  if (window_)
+    return window_;
+
+  running_ = true;
+  message_loop_runner_ = new content::MessageLoopRunner;
+  message_loop_runner_->Run();
+
+  EXPECT_TRUE(window_);

[bartfab (slow), 2014/03/25 12:52:54]

Nit: I think it would be better to let individual tests check for this
themselves. Putting gtest statements in helper classes is not a pattern I have
seen anywhere else and may surprise many test authors.

[rkc, 2014/03/26 21:30:40]

Done.

+  return window_;
+}
+
+void AppWindowWaiter::OnAppWindowAdded(apps::AppWindow* app_window) {
+  if (!running_)
+    return;
+
+  if (app_window->extension_id() == app_id_) {
+    window_ = app_window;
+    message_loop_runner_->Quit();
+    running_ = false;

[bartfab (slow), 2014/03/25 12:52:54]

Nit: If you just did message_loop_runner_ = NULL here, you could do away with
the explicit |running_| variable because:

|running_| == |message_loop_runner_|

[rkc, 2014/03/26 21:30:40]

Using run_loop_->running() instead.

Done.

+  }
+}
+
+void AppWindowWaiter::OnAppWindowIconChanged(apps::AppWindow* app_window) {}
+
+void AppWindowWaiter::OnAppWindowRemoved(apps::AppWindow* app_window) {}
+
+}  // namespace chromeos