Files app: Add division line for MyFiles on directory tree

Division line got lost when we enabled MyFilesVolume flag which made
MyFiles to have the default section. The division line is displayed when
a new section starts in the directory tree.

Change the test that checks for section to run with the flag enabled,
code used when flag is disabled will be removed soon after M77 branch.

Bug: 985415
Change-Id: I50957703426579b4203c47b88ac57f6b6c16dacd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1709313
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Austin Tankiang <austinct@chromium.org>
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Austin Tankiang <austinct@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678937}

ui/file_manager/file_manager/foreground/js/navigation_list_model.js

@@ -582,6 +582,7 @@ class NavigationListModel extends cr.EventTarget {
           myFilesModel = new NavigationModelFakeItem(
               str('MY_FILES_ROOT_LABEL'), NavigationModelItemType.ENTRY_LIST,
               myFilesEntry);
+          myFilesModel.section = NavigationSection.MY_FILES;
           this.myFilesModel_ = myFilesModel;
         } else {
           // When MyFilesVolume isn't available we create a empty EntryList to
@@ -593,6 +594,7 @@ class NavigationListModel extends cr.EventTarget {
           myFilesModel = new NavigationModelFakeItem(
               myFilesEntry.label, NavigationModelItemType.ENTRY_LIST,
               myFilesEntry);
+          myFilesModel.section = NavigationSection.MY_FILES;
         }
       } else {
         // Here is the initial version for MyFiles, which is only an entry in JS

ui/file_manager/file_manager/foreground/js/navigation_list_model_unittest.js

@@ -285,6 +285,9 @@ function testAddAndRemoveVolumes() {
  * 3. keeps MTP/Archive/Removable volumes on the original order.
  */
 function testOrderAndNestItems() {
+  // Enable My files.
+  loadTimeData.data_['MY_FILES_VOLUME_ENABLED'] = true;
+
   const volumeManager = new MockVolumeManager();
 
   const shortcutListModel = new MockFolderShortcutDataModel([