update_engine: Process Omaha response for rollback images

Omaha returns whether the image returned is a rollback in the
_rollback="true" argument. If this is set, the client has to check
whether it's OK to apply the rollback image (policy is specifically
requesting a rollback and verified boot will accept the image based
on its kernel and firmware key versions).

In addition to this, the device has to do a safe powerwash if the
image is a rollback. (We're not supporting rollbacks with partial
or no powerwash yet.)

We're also setting the rollback_happened preference to avoid force
updates happening before the policy is available again.

Chromium CL adding the error code: http://crrev.com/c/1047866

BUG=chromium:840432
TEST='cros_run_unit_tests --board=caroline --packages update_engine'

Change-Id: I1436ca96211b2a8523e78bf83602ef8b6b525570
Reviewed-on: https://chromium-review.googlesource.com/1047610
Commit-Ready: Marton Hunyady <hunyadym@chromium.org>
Tested-by: Marton Hunyady <hunyadym@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/payload_consumer/postinstall_runner_action.cc b/payload_consumer/postinstall_runner_action.cc
index cedecda..c672fef 100644
--- a/payload_consumer/postinstall_runner_action.cc
+++ b/payload_consumer/postinstall_runner_action.cc
@@ -57,7 +57,8 @@
   CHECK(HasInputObject());
   install_plan_ = GetInputObject();
 
-  if (install_plan_.powerwash_required) {
+  // Currently we're always powerwashing when rolling back.
+  if (install_plan_.powerwash_required || install_plan_.is_rollback) {
     if (hardware_->SchedulePowerwash()) {
       powerwash_scheduled_ = true;
     } else {