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Previously, 'Learner' was a roll-up trainer + model, intended to
make it easy for clients to use the learning system.  However, since
LearningSession can do this too, it makes sense to split the model
apart from the particular training algorithm used to build it.

Change-Id: If207bf64c6a5b34fb84591b9b149bdd9a3ff6af4
Reviewed-on: https://chromium-review.googlesource.com/c/1327544Reviewed-by: Fredrik Hubinette <hubbe@chromium.org>
Commit-Queue: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609802}