Update Mojo JS externs to use @record instead of @typedef.

This ensures that adding fields to Mojo structs which are not defined
within the struct's definition causes a compiler error.

Fixed: 1031385
Change-Id: I56cc509a192be882227763f47cec05ef6a8c5ad5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2006413
Auto-Submit: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#742653}

mojo/public/tools/bindings/generators/js_templates/lite/struct_definition.tmpl

@@ -41,17 +41,14 @@ mojo.internal.Struct(
 {%  if generate_closure_exports -%}
 goog.provide('{{module.namespace}}.{{struct.name}}');
 
-{% if struct.packed.packed_fields %}
-/**
- * @typedef { {
+/** @record */
+{{module.namespace}}.{{struct.name}} = class {
+  constructor() {
 {%- for packed_field in struct.packed.packed_fields %}
- *   {{packed_field.field.name}}: {{packed_field.field.kind|lite_closure_field_type}},
+    /** @type { {{packed_field.field.kind|lite_closure_field_type}} } */
+    this.{{packed_field.field.name}};
 {%-  endfor %}
- * } }
- */
-{% else %}
-/** @typedef {Object} */
-{% endif %}
-{{module.namespace}}.{{struct.name}};
+  }
+};
 
 {%- endif %}