Make better use of MediaTypeNames

These places may as well just use the global strings
from MediaTypeNames.

This CL also removes the unused speechButton method.

Review URL: https://codereview.chromium.org/564133003

git-svn-id: svn://svn.chromium.org/blink/trunk@182012 bbb929c8-8fbe-4397-9dbb-9b2b20218538