Fix test driver json output.

TBR=tandrii@chromium.org

Review URL: https://codereview.chromium.org/716713002

Cr-Commit-Position: refs/heads/master@{#25254}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25254 ce2b1a6d-e550-0410-aec6-3dcde31c8c00