[Net] Add JSON to kSecondaryMappings in mime_util

Add JSON as a known mime type in kSecondaryMappings in mime_util.
It's not quite common enough to put in kPrimaryMappings;
kSecondaryMappings will still allow local configurations to
apply on file:// urls.

Bug: 797712
Change-Id: Ia4c8ab8930a2d14da51ad781094df27d9a21afb5
Reviewed-on: https://chromium-review.googlesource.com/889915
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Asanka Herath <asanka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532733}
3 files changed