Add a Default instance of ptracestore, will be needed for ingestion.
Add a Default instance of ptracestore, will be needed for ingestion.

BUG=skia:

Review-Url: https://codereview.chromium.org/2288213002

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