gold: Fix byblame results to only include the gm corpus.
gold: Fix byblame results to only include the gm corpus.

In the longer run the corpus should be selectable from byblame.

BUG=skia:
TBR=mtklein

Review URL: https://codereview.chromium.org/1225583013

google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot