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This CL adds the ability to register a LearningTask with a
LearningSessionImpl.  It, in turn, creates a LearningTaskController
for that task.

The LearningTaskController is responsible for taking training
examples, and forwarding them to learner(s), along with feature
selection and accuracy reporting.

Change-Id: I071dd08063b1ff2482ca4e9170f7f2d0cb6bd10a
Reviewed-on: https://chromium-review.googlesource.com/c/1315934
Commit-Queue: Frank Liberato <liberato@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609127}