GTTF: Pass a flag for JSON machine-readable output in nacl buildbot scripts.

This depends on https://codereview.chromium.org/85923004/

BUG=317931

Review URL: https://codereview.chromium.org/85373005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237512 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237512