Fix Interface JS externs generation for mojo.

This CL fixes an issue with JS externs generation for Interface types
in mojo. This was blocking
https://chromium-review.googlesource.com/c/chromium/src/+/1053381.

Bug: 632206
Change-Id: I164bc3d9e9b0fd43f438dfefeaa2bc6e731fd1e7
Reviewed-on: https://chromium-review.googlesource.com/1053332Reviewed-by: Sam McNally <sammc@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558637}

mojo/public/tools/bindings/generators/mojom_js_generator.py

@@ -332,7 +332,9 @@ class Generator(generator.Generator):
   def _ClosureType(self, kind):
     if kind in mojom.PRIMITIVES:
       return _kind_to_closure_type[kind]
-    if (mojom.IsStructKind(kind) or mojom.IsInterfaceKind(kind) or
+    if mojom.IsInterfaceKind(kind):
+      return kind.module.namespace + "." + kind.name + "Ptr"
+    if (mojom.IsStructKind(kind) or
         mojom.IsEnumKind(kind)):
       return kind.module.namespace + "." + kind.name
     # TODO(calamity): Support unions properly.