Sky: Add a test for the <iframe> element

sky/viewer/document_view.cc

Index: sky/viewer/document_view.cc
diff --git a/sky/viewer/document_view.cc b/sky/viewer/document_view.cc
index 1d9bdea793bd2585e4d86acfabacb75e44c8212f..990ce5127f3660aa9e475eda7d1f68750f8e246c 100644
--- a/sky/viewer/document_view.cc
+++ b/sky/viewer/document_view.cc
@@ -218,9 +218,6 @@ void DocumentView::OnViewBoundsChanged(mojo::View* view,
 void DocumentView::OnViewDestroyed(mojo::View* view) {
   DCHECK_EQ(view, root_);
 
-  for (auto& child : root_->children())
-    child->Destroy();
-
   delete this;
 }