Allow set-wallpaper command in the toolbar button menu.

Bug: 740842
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I8a0826a93cb36588bda03b5acfbf633c35d8d11e
Reviewed-on: https://chromium-review.googlesource.com/577978Reviewed-by: Naoki Fukino <fukino@chromium.org>
Commit-Queue: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488161}

ui/file_manager/file_manager/foreground/js/file_manager_commands.js

@@ -1607,7 +1607,7 @@ CommandHandler.COMMANDS_['set-wallpaper'] = /** @type {Command} */ ({
     });
   },
   canExecute: function(event, fileManager) {
-    var entries = CommandUtil.getCommandEntries(event.target);
+    var entries = fileManager.getSelection().entries;
     if (entries.length === 0) {
       event.canExecute = false;
       event.command.setHidden(true);