Make ChromeOS screenshots use async readback path

ui/snapshot/snapshot_ios.mm

 // Copyright (c) 2012 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/snapshot.h"
 
 #include "base/callback.h"
 #include "ui/gfx/rect.h"
 
 namespace ui {
@@ skipping 18 matching lines @@
     const gfx::Size& target_size,
     scoped_refptr<base::TaskRunner> background_task_runner,
     GrabWindowSnapshotAsyncCallback callback) {
   NOTIMPLEMENTED();
 }
 
 void GrabWindowSnapshotAsync(
     gfx::NativeWindow window,
     const gfx::Rect& source_rect,
     scoped_refptr<base::TaskRunner> background_task_runner,
-    const GrabWindowSnapshotAsyncCallback& callback) {
+    const GrabWindowSnapshotAsyncPNGCallback& callback) {
   NOTIMPLEMENTED();
 }
 
 }  // namespace ui