Download via HTTP only if enterprise policy allows.

In order to rollout HTTP-downloads for AU to stable channel, we want to
be a bit more conservative to preseve the defense in depth we have now
with HTTPS. So, we're introduced a new enterprise policy which should be
explicitly enabled in order for the payloads to be downloaded via HTTP.

This CL adds the support for honoring such a policy in update engine.

BUG=chromium:235562
TEST=New unit tests added, existing ones updated and they all pass.
TEST=Tested on ZGB with and without policy and it works as expected.
Change-Id: I356efbe237b10031161a57c70cb851c521915a76
Reviewed-on: https://gerrit.chromium.org/gerrit/55805
Reviewed-by: Chris Sosa <sosa@chromium.org>
Tested-by: Jay Srinivasan <jaysri@chromium.org>
Commit-Queue: Jay Srinivasan <jaysri@chromium.org>
diff --git a/system_state.cc b/system_state.cc
index da18500..2f7d0dd 100644
--- a/system_state.cc
+++ b/system_state.cc
@@ -64,4 +64,8 @@
   return file_util::PathExists(FilePath(kOOBECompletedMarker));
 }
 
+bool RealSystemState::IsOfficialBuild() {
+  return utils::IsOfficialBuild();
+}
+
 }  // namespace chromeos_update_engine