Change file_manager_commands.js to use Polymer.

It currently uses cr.ui.commands, which can't be called asynchronously (canExecute has to be synchronous). This means we can't check things against the API before enabling them (e.g. for permissions checks).

Look at making something similar to command_manager.js in bookmarks:
https://cs.chromium.org/chromium/src/chrome/browser/resources/md_bookmarks/command_manager.js?sq=package:chromium&dr