GitLab

Replace LearningTaskControllerImpl's do-nothing initial
implementation with one that:
  - constructs the training callback based on the learning task
  - collects training examples as they're added
  - re-train the model periodically.
  - record stats about model performance

Change-Id: I6a949e6188c71d00be5449dc9539508254c7d74a
Reviewed-on: https://chromium-review.googlesource.com/c/1355434
Commit-Queue: Frank Liberato <liberato@chromium.org>
Reviewed-by: Fredrik Hubinette <hubbe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612860}