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We need to be able to simulate the removal of a characteristic.  This
addition to the interface seems to be the most straightforward way of
accomplishing this goal.

BUG=569709

Change-Id: If700ea99a6df9badc98fc81ef5f7461e574b7985
Reviewed-on: https://chromium-review.googlesource.com/583640Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Vincent Scheib <scheib@chromium.org>
Commit-Queue: Conley Owens <cco3@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491097}