RFC 8081 declares a top level font media type for all types of fonts.
Updating the mime types in sniffer to reflect the new changes.

Fixes #24524

Change-Id: Iba6cef4c5974e9930e14705720d42550ee87ba56
Reviewed-on: https://go-review.googlesource.com/102458
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>