Add GrabWindowSnapshotAsync tests

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 bf05a33c586b37c32347400eff4756dbccd19db7..c90450035e31ce6cc1a20728a0a2ce6512e4545d 100644
--- a/ui/snapshot/snapshot_gtk.cc
+++ b/ui/snapshot/snapshot_gtk.cc
@@ -83,7 +83,7 @@ bool GrabWindowSnapshot(gfx::NativeWindow window_handle,
       snapshot_bounds);
 }
 
-SNAPSHOT_EXPORT void GrabWindowSnapshotAsync(
+void GrabWindowSnapshotAndScaleAsync(
     gfx::NativeWindow window,
     const gfx::Rect& snapshot_bounds,
     const gfx::Size& target_size,
@@ -92,4 +92,12 @@ SNAPSHOT_EXPORT void GrabWindowSnapshotAsync(
   NOTIMPLEMENTED();
 }
 
+void GrabWindowSnapshotAsync(
+    gfx::NativeWindow window,
+    const gfx::Rect& source_rect,
+    scoped_refptr<base::TaskRunner> background_task_runner,
+    const GrabWindowSnapshotAsyncCallback& callback) {
+  NOTIMPLEMENTED();
+}
+
 }  // namespace ui