Use JSON as startup parameters to BufferSubData test. BUG=angle:744 Change-Id: I60287f3c612011f505f1bc20632efc4dd17001d3 Reviewed-on: https://chromium-review.googlesource.com/221375 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Brandon Jones <bajones@chromium.org>