Use indices into the vectors shared by runs

With multiple runs, the shared vectors were moving out from
under the pointers in earlier runs.

Change-Id: I486d2e603e18ea7effc0dbdbc7c5d3c545278703
Reviewed-on: https://skia-review.googlesource.com/137222
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
5 files changed