Recognize json media types as (non-image) mime types.

Extend the IsSupportedNonImageMimeType() predicate to also
recognize the JSON media types ("application/<something>+json")
( http://tools.ietf.org/html/rfc6839#section-3.1 )

Note that the 6839 RFC text allows the use of any toplevel type, not just "application/". No known uses outside of application/  however, hence we limit ourselves for now.

R=
BUG=278568

Review URL: https://codereview.chromium.org/97913002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242008 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242008