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Since it turns out to be much easier to record results for a
regressor than a classifier, add support for it to ExtraTrees.

Change-Id: I0734c81bafd87e06c23df8f74b7a0c4c80d616e6
Reviewed-on: https://chromium-review.googlesource.com/c/1391327
Commit-Queue: Frank Liberato <liberato@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619540}