auto merge of #3666 : ttaubert/servo/issue/3644-privatize-dom, r=Manishearth

This PR removes public fields from all (hope I didn't miss any) DOM structs. Should |Page| be privatized as well? This PR additionally introduces a #[privatize] lint to ensure nobody accidentally re-introduces a public field.

All changesets compile separately if applied in the same order. Hope that helps reviewing but I can of course squash them before merging.

components/script/dom/bindings/codegen/CodegenRust.py

@@ -5452,7 +5452,7 @@ class GlobalGenRoots():
                 protoDescriptor = config.getDescriptor(protoName)
                 delegate = string.Template('''impl ${selfName} for ${baseName} {
   fn ${fname}(&self) -> bool {
-    self.${parentName}.${fname}()
+    self.${parentName}().${fname}()
   }
 }
 ''').substitute({'fname': 'is_' + name.lower(),