Files app: Enable main_modules.html behind the flag

Bug: 1133186
Change-Id: Ieebd11507009669b138584be6a09d31e4ce20a7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2640316Reviewed-by: Jeremie Boulic <jboulic@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845532}

ui/file_manager/file_manager/background/js/launcher.js

@@ -173,8 +173,11 @@ launcher.launchFileManager = async (opt_appState, opt_id, opt_type) => {
     FILE_MANAGER_WINDOW_CREATE_OPTIONS.frame.color = '#ffffff';
   }
 
-  const appWindow = new AppWindowWrapper(
-      'main.html', appId, FILE_MANAGER_WINDOW_CREATE_OPTIONS);
+  const htmlFile =
+      util.isFilesJsModulesEnabled() ? 'main_modules.html' : 'main.html';
+  const appWindow =
+      new AppWindowWrapper(htmlFile, appId, FILE_MANAGER_WINDOW_CREATE_OPTIONS);
+
   await appWindow.launch(opt_appState || {}, false);
   if (!appWindow.rawAppWindow) {
     return null;