Make WindowReachedScreen support async readback

ui/snapshot/snapshot_gtk.cc

Index: ui/snapshot/snapshot_gtk.cc
diff --git a/ui/snapshot/snapshot_gtk.cc b/ui/snapshot/snapshot_gtk.cc
index 82598d9ce2d1930fd5e2787f8b04fd01da1edc0b..2ec86f3f8c523ddb65ff1ce095e85cfeb8ea1e5f 100644
--- a/ui/snapshot/snapshot_gtk.cc
+++ b/ui/snapshot/snapshot_gtk.cc
@@ -10,6 +10,7 @@
 #include "base/callback.h"
 #include "base/logging.h"
 #include "ui/base/x/x11_util.h"
+#include "ui/gfx/image/image.h"
 #include "ui/gfx/rect.h"
 
 namespace {
@@ -89,7 +90,15 @@ 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(
@@ -97,7 +106,7 @@ 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