Change from ES6 Map to old-fashioned object

This fixes the Kokoro tests broken by []

Unfortunately this means that the return type changes from a Map to an Object as well,
so all of the .get() lines need to turn into [].  Yay weakly typed languages. And
apparently in some error conditions this method could already return the Object type
anyway, even if you passed in a Map, so it's just poorly designed.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=213808477

java/google/registry/ui/js/session.js

@@ -48,13 +48,14 @@ registry.Session = function(defaultUri, xsrfToken, contentType) {
 
   /**
    * XHR request headers.
-   * @private {!Map.<string, string>}
+   * @private {!Object<string, string>}
    * @const
    */
-  this.headers_ = new Map([
-      ['Content-Type', contentType],
-      ['X-CSRF-Token', xsrfToken],
-      ['X-Requested-With', 'XMLHttpRequest']]);
+  this.headers_ = {
+    'Content-Type': contentType,
+    'X-CSRF-Token': xsrfToken,
+    'X-Requested-With': 'XMLHttpRequest'
+  };
 };