Move app_shell specific impl from athena_lib to athena_shell_lib

athena/content/shell/shell_app_activity.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/shell/shell_app_activity.h"
+
+#include "extensions/shell/browser/shell_app_window.h"
+
+namespace athena {
+
+ShellAppActivity::ShellAppActivity(extensions::ShellAppWindow* app_window)
+    : shell_app_window_(app_window) {
+}
+
+ShellAppActivity::~ShellAppActivity() {}
+
+content::WebContents* ShellAppActivity::GetWebContents() {
+  return shell_app_window_->GetAssociatedWebContents();
+}
+
+}  // namespace athena