pre-upload: split json check into a hook

BUG=chromium:466264
TEST=`./pre-upload_unittest.py` passes

Change-Id: I022809a1db3c4d48a8b3138a9453c39770303f0e
Reviewed-on: https://chromium-review.googlesource.com/262013
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Trybot-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Steve Fung <stevefung@chromium.org>
1 file changed