Make WindowReachedScreen support async readback

ui/snapshot/snapshot_ios.mm

Index: ui/snapshot/snapshot_ios.mm
diff --git a/ui/snapshot/snapshot_ios.mm b/ui/snapshot/snapshot_ios.mm
index 6637d28d7b2fea6a0c8ce8ec1fd91fa74cecda43..872b486512368616e2cf77df4656dbbf9fd12fcb 100644
--- a/ui/snapshot/snapshot_ios.mm
+++ b/ui/snapshot/snapshot_ios.mm
@@ -5,6 +5,7 @@
 #include "ui/snapshot/snapshot.h"
 
 #include "base/callback.h"
+#include "ui/gfx/image/image.h"
 #include "ui/gfx/rect.h"
 
 namespace ui {
@@ -29,7 +30,15 @@ void GrabWindowSnapshotAndScaleAsync(
     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(
@@ -37,7 +46,7 @@ void GrabWindowSnapshotAsync(
     const gfx::Rect& source_rect,
     scoped_refptr<base::TaskRunner> background_task_runner,
     const GrabWindowSnapshotAsyncPNGCallback& callback) {
-  NOTIMPLEMENTED();
+  callback.Run(scoped_refptr<base::RefCountedBytes>());
 }
 
 }  // namespace ui