add --descriptions flag to render_pictures tool

Needed because right now, when you look at the full set of SKP results in
rebaseline_server, you can't tell which renderMode (or builder) generated each
one.

BUG=skia:2833
R=borenet@google.com

Author: epoger@google.com

Review URL: https://codereview.chromium.org/466153006
5 files changed