Make the root_namespace argument to json_schema_compiler.gypi a string

ui/accessibility/BUILD.gn

Index: ui/accessibility/BUILD.gn
diff --git a/ui/accessibility/BUILD.gn b/ui/accessibility/BUILD.gn
index 1eac57676cb24ab23bc386d5546fba9e36ecdd03..32a4f71bd49c37dd55d4b11f99b522da4bbf1bb9 100644
--- a/ui/accessibility/BUILD.gn
+++ b/ui/accessibility/BUILD.gn
@@ -76,7 +76,7 @@ test("accessibility_unittests") {
 generated_extensions_api("ax_gen") {
   sources = [ "ax_enums.idl" ]
   deps = [ "//base/third_party/dynamic_annotations" ]
-  root_namespace = ""
+  root_namespace = "ui"
   impl_dir = "."
   bundle = true
 }