MediaCapabilities integration for local learning.

This change adds the first local learning experiment for
MediaCapabilities.  If the MediaLearningExperiment switch is on,
then a local learner will log a confusion matrix via UMA for
MediaCapabilities predictions.

No behavior change is expected; the results of learning are used for
logging only.

Change-Id: I0b99ed6d6d30d58acb1327e6bcf76207d2524dad
Reviewed-on: https://chromium-review.googlesource.com/c/1394736
Commit-Queue: Frank Liberato <liberato@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625023}
20 files changed