Add PredictorFactory.

The PredictorFactory contains all the predictors names and types constants.
The factory proposes two static methods:
1. A method to get a PredictorType when passing a predictor name
2. A method to get a unique instance of a predictor when passing a
   PredictorType

Each predictor now returns the constant stored in the factory when
calling GetName()

Bug: 981147
Change-Id: Ib1069b069d8eb4fb45313aa4b9fbddd49f8bf84e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1688283
Reviewed-by: Navid Zolghadr <nzolghadr@chromium.org>
Reviewed-by: Ella Ge <eirage@chromium.org>
Commit-Queue: Axel Antoine <axantoine@google.com>
Cr-Commit-Position: refs/heads/master@{#675857}
14 files changed