Support for json media types as (non-image) mime types.

Consider mime types with a JSON media type (*) just like
"application/json" is; i.e., rendered as text content. Similarly for
devtools, have such mime types be equally previewable. Also addressed
a smaller bug which made non-XHR loaded resources with text mime types
(but no charset supplied in the response) to not be previewable.

Added inspector tests to verify that such mime types are indeed
previewed using the JSON viewer.

* - "application/<something>+json", http://tools.ietf.org/html/rfc6839#section-3.1

R=tyoshino@chromium.org,pfeldman@chromium.org,vsevik@chromium.org
BUG=278568

Review URL: https://codereview.chromium.org/94893003

git-svn-id: svn://svn.chromium.org/blink/trunk@164220 bbb929c8-8fbe-4397-9dbb-9b2b20218538