GitLab

For clients in a Precache trial group who've started a precache in the last 15
days, registers them in a matching group in the PrecacheSynthetic15D trial. For
those who've started a precache in the last 1 day, registers in them in the
PrecacheSynthetic1D trial also.

BUG=663987

Review-Url: https://codereview.chromium.org/2596093002
Cr-Commit-Position: refs/heads/master@{#443352}