Make WindowReachedScreen support async readback

ui/snapshot/snapshot_mac.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"
 
 #import <Cocoa/Cocoa.h>
 
 #include "base/callback.h"
 #include "base/logging.h"
 #include "base/mac/scoped_cftyperef.h"
 #include "base/mac/scoped_nsobject.h"
+#include "base/task_runner.h"
+#include "ui/gfx/image/image.h"
 #include "ui/gfx/rect.h"
 
 namespace ui {
 
 bool GrabViewSnapshot(gfx::NativeView view,
                       std::vector<unsigned char>* png_representation,
                       const gfx::Rect& snapshot_bounds) {
   NSWindow* window = [view window];
   NSScreen* screen = [[NSScreen screens] objectAtIndex:0];
   gfx::Rect screen_bounds = gfx::Rect(NSRectToCGRect([screen frame]));
@@ skipping 49 matching lines @@
   return GrabViewSnapshot([[window contentView] superview], png_representation,
       snapshot_bounds);
 }
 
 void GrabWindowSnapshotAndScaleAsync(
     gfx::NativeWindow window,
     const gfx::Rect& snapshot_bounds,
     const gfx::Size& target_size,
     scoped_refptr<base::TaskRunner> background_task_runner,
     GrabWindowSnapshotAsyncCallback callback) {
-  NOTIMPLEMENTED();
+  callback.Run(gfx::Image());
+}
+
+void GrabViewSnapshotAsync(
+    gfx::NativeView view,
+    const gfx::Rect& source_rect,
+    scoped_refptr<base::TaskRunner> background_task_runner,
+    const GrabWindowSnapshotAsyncPNGCallback& callback) {
+  callback.Run(scoped_refptr<base::RefCountedBytes>());
 }
 
 void GrabWindowSnapshotAsync(
     gfx::NativeWindow window,
     const gfx::Rect& source_rect,
     scoped_refptr<base::TaskRunner> background_task_runner,
     const GrabWindowSnapshotAsyncPNGCallback& callback) {
-  NOTIMPLEMENTED();
+  return GrabViewSnapshotAsync([[window contentView] superview], source_rect,
+      background_task_runner, callback);
 }
 
 }  // namespace ui