https://tools.ietf.org/html/rfc7303 says that if "new media type is
introduced for an XML-based format, the name of the media type SHOULD
end with '+xml'".

https://tools.ietf.org/html/rfc6839 covers '+xml' and '+json' suffixes.

https://mimesniff.spec.whatwg.org/#xml-mime-type says "An XML MIME type
is any MIME type whose subtype ends in '+xml' or whose essence is
'text/xml' or 'application/xml'. [RFC7303]".

https://mimesniff.spec.whatwg.org/#json-mime-type says "A JSON MIME type
is any MIME type whose subtype ends in '+json' or whose essence is
'application/json' or 'text/json'."

There are no occurences of "application/xml+", "text/xml+",
"application/json+", "text/json+" or "text/x-json" in the specs above
and on various lists of MIME types like:
-
https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types/Complete_list_of_MIME_types
- https://en.wikipedia.org/wiki/Media_type
- https://www.freeformatter.com/mime-types-list.html
- https://www.sitepoint.com/mime-types-complete-list/

Bug: 826756
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: Ied30f9728bd4f082bb620fea150f342457ea4833
Reviewed-on: https://chromium-review.googlesource.com/985211
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Nick Carter <nick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547565}