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..5ee3dfaaeb0a4987e1a6aba5215a872bb6e3c676 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
 }