[Sky] Fix iOS build breaks

sky/shell/ios/sky_surface.mm

Index: sky/shell/ios/sky_surface.mm
diff --git a/sky/shell/ios/sky_surface.mm b/sky/shell/ios/sky_surface.mm
index 518c9cc3b5e12d166aa9da12408ed4bc3c5c553b..1cb1fe0d5468291a6f94d1c9268e3291fc5bb8fa 100644
--- a/sky/shell/ios/sky_surface.mm
+++ b/sky/shell/ios/sky_surface.mm
@@ -86,7 +86,7 @@ static sky::InputEventPtr BasicInputEventFromRecognizer(
   CGSize size = self.bounds.size;
   CGFloat scale = [UIScreen mainScreen].scale;
 
-  ViewportMetricsPtr metrics = ViewportMetrics::New();
+  sky::ViewportMetricsPtr metrics = sky::ViewportMetrics::New();
   metrics->physical_width = size.width * scale;
   metrics->physical_height = size.height * scale;
   metrics->device_pixel_ratio = scale;