Move app_shell specific impl from athena_lib to athena_shell_lib

athena/content/shell/content_activity_factory.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 "athena/content/content_activity_factory.h"
+
+#include "athena/content/shell/shell_app_activity.h"
+
+namespace athena {
+
+Activity* ContentActivityFactory::CreateAppActivity(
+    extensions::ShellAppWindow* app_window) {
+  return new ShellAppActivity(app_window);
+}
+
+Activity* ContentActivityFactory::CreateAppActivity(
+    apps::AppWindow* app_window) {
+  return NULL;
+}
+
+}  // namespace athena