cr.js: Replace usage of ES6 Map with Object.

ui/webui/resources/js/cr.js

Index: ui/webui/resources/js/cr.js
diff --git a/ui/webui/resources/js/cr.js b/ui/webui/resources/js/cr.js
index ceb1eb9707474fb9d91f9bc6b12ad5b13858569f..a38c59f84732e820c00407287b3ef00c37b41bb2 100644
--- a/ui/webui/resources/js/cr.js
+++ b/ui/webui/resources/js/cr.js
@@ -318,9 +318,9 @@ var cr = function() {
    * sent by the browser in response to the chrome.send call. The mapping is
    * from ID to callback function; the ID is generated by sendWithCallback and
    * is unique across all invocations of said method.
-   * @type {!Map<string, Function>}
+   * @type {!Object<Function>}
    */
-  var chromeSendCallbackMap = new Map();
+  var chromeSendCallbackMap = Object.create(null);
 
   /**
    * The named method the WebUI handler calls directly in response to a
@@ -335,9 +335,9 @@ var cr = function() {
    *    response is tied to.
    */
   function webUIResponse(id) {
-    chromeSendCallbackMap.get(id).apply(
+    chromeSendCallbackMap[id].apply(
         null, Array.prototype.slice.call(arguments, 1));
-    chromeSendCallbackMap.delete(id);
+    delete chromeSendCallbackMap[id];
   }
 
   /**
@@ -353,16 +353,16 @@ var cr = function() {
    */
   function sendWithCallback(methodName, args, callback) {
     var id = methodName + createUid();
-    chromeSendCallbackMap.set(id, callback);
+    chromeSendCallbackMap[id] = callback;
     chrome.send(methodName, ['cr.webUIResponse', id].concat(args || []));
   }
 
   /**
    * A registry of callbacks keyed by event name. Used by addWebUIListener to
    * register listeners.
-   * @type {!Map<string, Array<Function>>}
+   * @type {!Object<Array<Function>>}
    */
-  var webUIListenerMap = new Map();
+  var webUIListenerMap = Object.create(null);
 
   /**
    * The named method the WebUI handler calls directly when an event occurs.
@@ -372,7 +372,7 @@ var cr = function() {
    * @param {string} event The name of the event that has occurred.
    */
   function webUIListenerCallback(event) {
-    var listenerCallbacks = webUIListenerMap.get(event);
+    var listenerCallbacks = webUIListenerMap[event];
     for (var i = 0; i < listenerCallbacks.length; i++) {
       var callback = listenerCallbacks[i];
       callback.apply(null, Array.prototype.slice.call(arguments, 1));
@@ -386,10 +386,10 @@ var cr = function() {
    * @param {Function} callback The callback run when the event is fired.
    */
   function addWebUIListener(event, callback) {
-    if (!webUIListenerMap.has(event))
-      webUIListenerMap.set(event, [callback]);
+    if (event in webUIListenerMap)
+      webUIListenerMap[event].push(callback);
     else
-      webUIListenerMap.get(event).push(callback);
+      webUIListenerMap[event] = [callback];
   }
 
   return {