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This CL adds unused mojo bindings for LearningTaskController.  The
service side owns a LearningTaskController to which it forwards
calls.  It doesn't have to worry about tracking in-flight
observations since it just drops the LearningTaskController when the
connection to the client is dropped.

The client side just forwards requests to the service.  Both are
more or less just type adapters from ::mojom to the real types.

Change-Id: I707987c8ee2c8bff1915e58d0ae60b0752074674
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1513204Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642347}