[snippets-internals] Fix Mojo Closure externs generation.

This CL adds an Interface type for concrete implementations to @implement
and makes the Mojo interface non-nullable to avoid needing an assert.

Bug: 836128
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ie2f1d07966f734a47c8bc724a11641d0cab67daf
Reviewed-on: https://chromium-review.googlesource.com/1088355
Commit-Queue: calamity <calamity@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567160}

chrome/browser/resources/snippets_internals/snippets_internals.js

@@ -7,7 +7,7 @@
 /** @type {snippetsInternals.mojom.PageHandlerPtr} */
 let pageHandler = null;
 
-/** @type {snippetsInternals.mojom.PagePtr} */
+/** @type {snippetsInternals.mojom.PageImpl} */
 let page = null;
 
 /* Javascript module for chrome://snippets-internals. */
@@ -276,15 +276,11 @@ function setupEventListeners() {
 }
 
 /* Represents the js-side of the IPC link. Backend talks to this. */
-// TODO (calamity): Add a closure interface.
-class SnippetsInternalsPageImpl extends snippetsInternals.mojom.PagePtr {
+/** @implements {snippetsInternals.mojom.PageImpl} */
+class SnippetsInternalsPageImpl {
   constructor(request) {
-    super();
-
-    const page = snippetsInternals.mojom.Page;
-    assert(page !== null);
-
-    this.binding_ = new mojo.Binding(page, this, request);
+    this.binding_ =
+        new mojo.Binding(snippetsInternals.mojom.Page, this, request);
   }
 
   /* Callback for when suggestions change on the backend. */

mojo/public/tools/bindings/generators/js_templates/externs/interface_definition.tmpl

-/** @implements {mojo.InterfacePtr} */
-{{module.namespace}}.{{interface.name}}Ptr = class {
-{%  for method in interface.methods %}
+{% macro generateInterfaceClass() -%}
+class {
+{%-  for method in interface.methods %}
   /**
 {%-   for parameter in method.parameters %}
    * @param { {{parameter.kind|closure_type_with_nullability}} } {{parameter.name|sanitize_identifier}}
@@ -9,16 +9,26 @@
    * @return {Promise}
 {%-   endif %}
    */
-   {{method.name}}(
+  {{method.name}}(
 {%-   for parameter in method.parameters -%}
 {{parameter.name|sanitize_identifier}}{% if not loop.last %}, {% endif %}
 {%-   endfor -%}
 ) {}
 {%- endfor %}
 };
+{%- endmacro %}
 
 /**
  * @const
- * @type { mojo.Interface };
+ * @type { !mojo.Interface };
  */
 {{module.namespace}}.{{interface.name}};
+
+/** @interface */
+{{module.namespace}}.{{interface.name}}Impl = {{ generateInterfaceClass() }}
+
+/**
+ * @implements { mojo.InterfacePtr }
+ * @implements { {{module.namespace}}.{{interface.name}}Impl }
+ */
+{{module.namespace}}.{{interface.name}}Ptr = {{ generateInterfaceClass() }}