[directorytree] Add files-ng generic provided icons if needed

Every volume type in FilesApp has volumeMetadata from chrome.FMP, that
contains an iconSet property: it is empty (contains no icons) for most
volume types.

Somes volumes like AndroidAppItem, PROVIDED, and DOCUMENT_PROVIDER can
(and usually do) provide icons in the iconSet property. If they don't,
FilesApp draws nothing in their .tree-row .item-icon element today.

AndroidAppItem volumes can provide icons: in the files-ng case, draw a
generic provided icon if their iconSet has no icons.

VolumeItem PROVIDED and DOCUMENT_PROVIDER can provide icons, but there
is no reliable way to detect/fix their missing icons today since other
volume types use VolumeItem too. If they don't provide icons, FilesApp
files-ng will now draw a gray box in their .item-icon, a visual change
in behavior compared to normal FilesApp.

Bug: 992819
Change-Id: I6d2ec07ef91fd3acd8edff8bc5053eab4450680c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1945958Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Alex Danilo <adanilo@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721279}

ui/file_manager/file_manager/foreground/js/ui/directory_tree.js

@@ -1116,19 +1116,19 @@ class VolumeItem extends DirectoryItem {
    */
   setupIcon_(icon, volumeInfo) {
     icon.classList.add('item-icon');
+
     const backgroundImage =
         util.iconSetToCSSBackgroundImageValue(volumeInfo.iconSet);
     if (backgroundImage !== 'none') {
-      // The icon div is not yet added to DOM, therefore it is impossible to
-      // use style.backgroundImage.
       icon.setAttribute('style', 'background-image: ' + backgroundImage);
     }
+
     icon.setAttribute('volume-type-icon', volumeInfo.volumeType);
+
     if (volumeInfo.volumeType === VolumeManagerCommon.VolumeType.MEDIA_VIEW) {
-      icon.setAttribute(
-          'volume-subtype',
-          VolumeManagerCommon.getMediaViewRootTypeFromVolumeId(
-              volumeInfo.volumeId));
+      const subtype = VolumeManagerCommon.getMediaViewRootTypeFromVolumeId(
+          volumeInfo.volumeId);
+      icon.setAttribute('volume-subtype', subtype);
     } else {
       icon.setAttribute('volume-subtype', volumeInfo.deviceType || '');
     }
@@ -1676,6 +1676,10 @@ class AndroidAppItem extends TreeItem {
       }
     }
 
+    if (directorytree.FILES_NG_ENABLED && !icon.hasAttribute('style')) {
+      icon.setAttribute('use-generic-provided-icon', '');
+    }
+
     // Create an external link icon. TODO(crbug.com/986169) does this icon
     // element need aria-label, role, tabindex, etc?
     const externalLinkIcon = document.createElement('span');