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