libpolicy: Add a MockPolicyProvider.

This patch adds a mock of the PolicyProvider and makes all the
PolicyProvider methods virtual.

BUG=chromium:362370
TEST=emerge-link platform2; compiled/used mock in update_engine unittests.

Change-Id: Ic716c0b83ea2f72cedcf77defd08b1b27ff88e8f
Reviewed-on: https://chromium-review.googlesource.com/194493
Tested-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
3 files changed