update_payload: Add zero operation support

Currenlty the applier.py and checker.py does not support ZERO
operation. This patch adds support for it. In addition, when generating
ZERO operation, we were not clearing the source length and extent
properties out of the protobuf. ZERO operation has no source length or
extent.

BUG=chromium:768461
TEST=unittest pass; scripts/paycheck.py --check payload.delta;

Change-Id: I766deaca4380686797893c2686036d59525546f4
Reviewed-on: https://chromium-review.googlesource.com/713539


Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>