Fixes Web Share for both text/plain and files (#458)
Fixes Web Share for both text/plain and files (#458)

- When the shareTarget element contains both text/url/title and
  the files element, add "text/plain" and the mime-types from
  the files.
GoogleChromeLabs/bubblewrap,GoogleChromeLabs/bubblewrap,GoogleChromeLabs/bubblewrap,GoogleChromeLabs/bubblewrap