[MacViews] Remove stray/unguarded uses of ui/aura/window.h in cross-platform files

ui/snapshot/screenshot_grabber.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 "ui/snapshot/screenshot_grabber.h"
 
 #include <climits>
 #include <string>
 
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/files/file_util.h"
 #include "base/logging.h"
+#include "base/message_loop/message_loop.h"
 #include "base/task_runner.h"
 #include "base/threading/sequenced_worker_pool.h"
 #include "base/time/time.h"
 #include "ui/gfx/image/image.h"
 #include "ui/snapshot/snapshot.h"
 
 #if defined(USE_AURA)
 #include "ui/aura/window.h"
 #endif
 
@@ skipping 163 matching lines @@
 
   ShowNotificationCallback notification_callback(base::Bind(
       &ScreenshotGrabber::NotifyScreenshotCompleted, factory_.GetWeakPtr()));
   client_->PrepareFileAndRunOnBlockingPool(
       screenshot_path, blocking_task_runner_,
       base::Bind(&SaveScreenshot, base::MessageLoop::current()->task_runner(),
                  notification_callback, screenshot_path, png_data));
 }
 
 }  // namespace ui