[Mojo Lite] Expose closeBindings method in interfaces.

This CL exposes a way to close bindings on an interface so that the
client can explicitly disconnect and stop receiving messages.

Change-Id: I322f4e41d5522fe8aa7ba9b4a1eb702607281dfe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636850
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Azeem Arshad <azeemarshad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664795}

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

@@ -143,6 +143,15 @@ goog.provide('{{module.namespace}}.{{interface.name}}Request');
     this.target_.bindHandle(handle);
   }
 
+  /**
+   * Closes all bindings bound to this target.
+   *
+   * @export
+   */
+  closeBindings() {
+    this.target_.closeBindings();
+  }
+
   /**
    * Returns a proxy for this interface which sends messages to the browser.
    * The browser must have an interface request binder registered for this