Implementation of translation event logging.

Integration steps described in:

https://docs.google.com/document/d/1es1cCRFhdag4ljGSPfXfSMlEx5lMgFZ3SGHMaBcU_D8/edit#heading=h.qdctas2f6c6e

BUG=728491

Review-Url: https://codereview.chromium.org/2913593002
Cr-Commit-Position: refs/heads/master@{#479219}
15 files changed