update_engine: UM: MockPolicy extends DefaultPolicy by default.

This affords us an easily customizable policy implementation for use
during testing.

BUG=None
TEST=Unit tests.

Change-Id: I264d4992d552539ed8a1da64e500fb7a94b6e2d0
Reviewed-on: https://chromium-review.googlesource.com/226343
Reviewed-by: Alex Deymo <deymo@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
1 file changed