Files.app: Let Files.app update the availability of delete command.

Originally the availability is updated only if the selection has items.  But the
update is needed to hide trash icon even if the selection hs no item.

BUG=290634
TEST=manually

Review URL: https://codereview.chromium.org/26666002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227741 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227741