Only set a valid surface from html_viewer

We have users of html_viewer that do not use the mojo provided services, including the surface service. This change: https://codereview.chromium.org/869343004/ broke them

I think it's right to check this here. Another option would be to do it in layer_tree_host.cc

BUG=

Review URL: https://codereview.chromium.org/902293002

Cr-Commit-Position: refs/heads/master@{#315157}

mojo/services/html_viewer/weblayertreeview_impl.cc

@@ -101,7 +101,8 @@ void WebLayerTreeViewImpl::ApplyViewportDeltas(
 }
 
 void WebLayerTreeViewImpl::RequestNewOutputSurface() {
-  layer_tree_host_->SetOutputSurface(output_surface_.Pass());
+  if (output_surface_.get())
+    layer_tree_host_->SetOutputSurface(output_surface_.Pass());
 }
 
 void WebLayerTreeViewImpl::DidFailToInitializeOutputSurface() {