[dragdrop] Update setDropTarget_ comments and code

Use "directory" instead of "folder". Elaborate the Crostini comment re:
it is actually a fake entry. However, the reason why we need to change
immediately to its directory when it is a fake entry is not explained,
and is potentially no longer relevant now Crostini is controlled by an
enable flag looking at CL:2264086

No change in behavior, no new tests.

Bug: 1002394
Change-Id: Ida7177b052f6fcb8db47e6181ffc4c5d708604c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2275280Reviewed-by: Alex Danilo <adanilo@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784316}

ui/file_manager/file_manager/foreground/js/file_transfer_controller.js

@@ -995,12 +995,13 @@ class FileTransferController {
       return;
     }
 
-    // Set classes assuming domElement won't accept this drop.
+    assert(destinationEntry.isDirectory);
+
+    // Assume the destination directory won't accept this drop.
     domElement.classList.remove('accepts');
     domElement.classList.add('denies');
 
-    // Disallow dropping a folder on itself.
-    assert(destinationEntry.isDirectory);
+    // Disallow dropping a directory on itself.
     const entries = this.selectionHandler_.selection.entries;
     for (let i = 0; i < entries.length; i++) {
       if (util.isSameEntry(entries[i], destinationEntry)) {
@@ -1008,21 +1009,23 @@ class FileTransferController {
       }
     }
 
-    // Add accept class if the domElement can accept this drop.
+    this.destinationEntry_ = destinationEntry;
+
+    // Add accept classes if the directory can accept this drop.
     if (this.canPasteOrDrop_(clipboardData, destinationEntry)) {
       domElement.classList.remove('denies');
       domElement.classList.add('accepts');
     }
 
-    this.destinationEntry_ = destinationEntry;
-
-    // Change directory immediately for crostini, otherwise start timer.
+    // Change directory immediately if it's a fake entry for Crostini.
     if (destinationEntry.rootType === VolumeManagerCommon.RootType.CROSTINI) {
       this.changeToDropTargetDirectory_();
-    } else {
-      this.navigateTimer_ =
-          setTimeout(this.changeToDropTargetDirectory_.bind(this), 2000);
+      return;
     }
+
+    // Change to the directory after the drag target hover time out.
+    const navigate = this.changeToDropTargetDirectory_.bind(this);
+    this.navigateTimer_ = setTimeout(navigate, 2000);
   }
 
   /**