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This allows tests that want to provide mock implementations of these objects to
just override what's necessary to change functional behavior, not provide
boilerplate simply to instantiate the objects at all.

BUG=none
TEST=none

Change-Id: I0b3ccc6da2ae2b3a87789c06d427cae227a2d662
Reviewed-on: https://chromium-review.googlesource.com/987326
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547915}