vm_run_skia_try/write_json_summary: run skpdiff within imagediffdb, not on its own

This is a prerequisite for making write_json_summary use imagepairset, and thus
work with the same frontend UI as rebaseline_server.

BUG=skia:2230
NOTRY=True  # no C++ files in the CL
R=rmistry@google.com

Author: epoger@google.com

Review URL: https://codereview.chromium.org/183763025
6 files changed