[bindings] Add type checking annotations to NamedMessagePortConnector.

Adds annotations and a check-only js_binary() target to provide
static typechecking assurances for NamedMessagePortConnector's JS.

Bug: 159646269
Change-Id: I8340e4ef8ca291431a26dccef642745625e36911
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2458811
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Auto-Submit: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821805}

components/cast/named_message_port_connector/BUILD.gn

@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//third_party/closure_compiler/compile_js.gni")
 import("//tools/grit/grit_rule.gni")
 
 source_set("named_message_port_connector") {
@@ -21,6 +22,7 @@ source_set("named_message_port_connector") {
 
 grit("resources") {
   source = "named_message_port_connector_resources.grd"
+  deps = [ ":js_type_check" ]
   outputs = [
     "grit/named_message_port_connector_resources.h",
     "named_message_port_connector_resources.pak",
@@ -29,3 +31,9 @@ grit("resources") {
   # Allow GRIT to assign IDs using its default set of base IDs.
   resource_ids = ""
 }
+
+js_binary("js_type_check") {
+  sources = [ "named_message_port_connector.js" ]
+  checks_only = true
+  closure_flags = strict_error_checking_closure_args
+}

components/cast/named_message_port_connector/named_message_port_connector.js

@@ -4,34 +4,48 @@
 
 'use strict';
 
-if (!cast) {
+if (!window['cast']) {
+  /**
+   * @const
+   */
   // eslint-disable-next-line no-var
   var cast = new Object;
 }
 
-if (!cast.__platform__) {
-  cast.__platform__ = new Object;
+if (!cast['__platform__']) {
+  /**
+   * @const
+   */
+  cast.__platform__ = {};
 }
 
 // Creates named HTML5 MessagePorts that are connected to native code.
 cast.__platform__.PortConnector = new class {
   constructor() {
+    /** @private {MessagePort} */
     this.controlPort_ = null;
 
     // A map of ports waiting to be published to the controlPort_, keyed by
     // string IDs.
+    /** @private {Object<string, MessagePort>} */
     this.pendingPorts_ = {};
 
+    /** @private */
     this.listener = this.onMessageEvent.bind(this);
+
     window.addEventListener(
         'message', this.listener,
         true  // Let the listener handle events before they hit the DOM tree.
     );
   }
 
-  // Returns a MessagePort whose channel will be passed to the native code.
-  // The channel can be used immediately after construction. Outgoing messages
-  // will be automatically buffered until the connection is established.
+  /**
+   * Returns a MessagePort whose channel will be passed to the native code.
+   * The channel can be used immediately after construction. Outgoing messages
+   * will be automatically buffered until the connection is established.
+   * @param {string} id The ID of the port being registered.
+   * @return {MessagePort}
+   */
   bind(id) {
     const channel = new MessageChannel();
     if (this.controlPort_) {
@@ -43,33 +57,43 @@ cast.__platform__.PortConnector = new class {
     return channel.port1;
   }
 
+  /**
+   * Sends a MessagePort to the remote NamedMessagePortConnector.
+   * @param {string} portId The name of the port to send over the control port.
+   * @param {MessagePort} port The port being sent.
+   */
   sendPort(portId, port) {
     this.controlPort_.postMessage(portId, [port]);
   }
 
-  // Receives a control port from native code.
-  onMessageEvent(e) {
+  /**
+   * Handles frame message events to receive a connection "control port" from
+   * native code.
+   * @param {Event} messageEvent
+   */
+  onMessageEvent(messageEvent) {
     // Only process window.onmessage events which are intended for this class.
-    if (e.data != 'cast.master.connect') {
+    if (messageEvent.data != 'cast.master.connect') {
       return;
     }
 
-    if (e.ports.length != 1) {
+    if (messageEvent.ports.length != 1) {
       console.error(
-          'Expected only one MessagePort, got ' + e.ports.length + ' instead.');
-      for (const i in e.ports) {
-        e.ports[i].close();
+          'Expected only one MessagePort, got ' + messageEvent.ports.length +
+          ' instead.');
+      for (const port of messageEvent.ports) {
+        port.close();
       }
       return;
     }
 
-    this.controlPort_ = e.ports[0];
+    this.controlPort_ = messageEvent.ports[0];
     for (const portId in this.pendingPorts_) {
       this.sendPort(portId, this.pendingPorts_[portId]);
     }
     this.pendingPorts_ = null;
 
-    e.stopPropagation();
+    messageEvent.stopPropagation();
 
     // No need to receive more onmessage events.
     window.removeEventListener('message', this.listener);