[Files.app] Show nice error message when user clicks .crx file on Files.app

BUG=308347
TEST=manually tested

Review URL: https://codereview.chromium.org/100283004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239485 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239485