Files app: Display "Show hidden Play folders" conditionally.

The gear menu option "Show hidden Play folders" doesn't make sense when the
"Play Files" volume is not mounted.
The option should be visible only when the corresponding volume is mounted.

Bug: 851385
Test: Manually tested on Eve.
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I5f39a669890c455cbd9763a6d0d047854545acca
Reviewed-on: https://chromium-review.googlesource.com/1100721
Commit-Queue: Naoki Fukino <fukino@chromium.org>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567225}

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

@@ -795,7 +795,12 @@ CommandHandler.COMMANDS_['toggle-hidden-android-folders'] =
        * @param {!Event} event Command event.
        * @param {!CommandHandlerDeps} fileManager CommandHandlerDeps to use.
        */
-      canExecute: CommandUtil.canExecuteAlways
+      canExecute: function(event, fileManager) {
+        event.canExecute =
+            !!fileManager.volumeManager.getCurrentProfileVolumeInfo(
+                VolumeManagerCommon.VolumeType.ANDROID_FILES);
+        event.command.setHidden(!event.canExecute);
+      }
     });
 
 /**