Files app: Generate JS Module for //ui/file_manager/externs/background_window.js

Add JS module generation for:
//ui/file_manager/externs/background_window.js

Add the method forceFileOperationErrorForTest() in the interface
FileBrowserBackgroundFull and to its implementation
FileBrowserBackgroundFullImpl.  This is a preparation to run as JS
modules and as SWA; to avoid the integration test harness to reach to
global namespace `fileOperationUtil`.

Bug: 1133186
Change-Id: Ic5e2d38762b030c827c887021e7e611ba846228d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2537285
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Jeremie Boulic <jboulic@chromium.org>
Commit-Queue: Jeremie Boulic <jboulic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827714}

ui/file_manager/externs/BUILD.gn

@@ -18,6 +18,14 @@ js_library("file_manager_private") {
   ]
 }
 
+js_library("background_window.m") {
+  sources = [ "$root_gen_dir/ui/file_manager/externs/background_window.m.js" ]
+
+  deps = [ "//ui/file_manager/externs/background:background_base.m" ]
+
+  extra_deps = [ ":modulize" ]
+}
+
 js_library("entry_location.m") {
   sources = [ "$root_gen_dir/ui/file_manager/externs/entry_location.m.js" ]
 
@@ -95,6 +103,7 @@ js_library("volume_info_list.m") {
 
 js_modulizer("modulize") {
   input_files = [
+    "background_window.js",
     "entry_location.js",
     "files_app_entry_interfaces.js",
     "metadata_worker_window.js",

ui/file_manager/externs/background/file_browser_background_full.js

@@ -57,4 +57,10 @@ class FileBrowserBackgroundFull extends BackgroundBase {
    * @param {function()} callback
    */
   ready(callback) {}
+
+  /**
+   * Forces File Operation Util to return error for automated tests.
+   * @param {boolean} enable
+   */
+  forceFileOperationErrorForTest(enable) {}
 }

ui/file_manager/externs/background_window.js

@@ -2,11 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// #import {BackgroundBase} from './background/background_base.m.js';
+
 /**
- * @constructor
  * @extends {Window}
  */
-class BackgroundWindow {
+/* #export */ class BackgroundWindow {
   constructor() {
     /**
      * For File Manager it uses FileBrowserBackgroundFull.

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

@@ -140,6 +140,14 @@ class FileBrowserBackgroundImpl extends BackgroundBaseImpl {
     this.initializationPromise_.then(callback);
   }
 
+  /**
+   * Forces File Operation Util to return error for automated tests.
+   * @param {boolean} enable
+   */
+  forceFileOperationErrorForTest(enable) {
+    fileOperationUtil.forceErrorForTest = enable;
+  }
+
   /**
    * Opens the volume root (or opt directoryPath) in main UI.
    *