Make ui gn check pass. Misc GN build improvements.

extensions/generated_extensions_api.gni

Index: extensions/generated_extensions_api.gni
diff --git a/extensions/generated_extensions_api.gni b/extensions/generated_extensions_api.gni
index 793ec64a11726c86a397732492066e6e3dfd6b2f..81c5e86078212417757b467d8c6300b8f157460b 100644
--- a/extensions/generated_extensions_api.gni
+++ b/extensions/generated_extensions_api.gni
@@ -109,7 +109,11 @@ template("generated_extensions_api") {
         "--namespace=$root_namespace",
         "--generator=cpp",
         "--include-rules=$schema_include_rules" ]
-      visibility = target_visibility
+
+      if (defined(invoker.visibility)) {
+        # If visibility is restricted, add our own target to it.
+        visibility = [ invoker.visibility, target_visibility ]
+      }
     }
   }