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/real_system_state.h b/real_system_state.h
index 42d1509..83f3ac1 100644
--- a/real_system_state.h
+++ b/real_system_state.h
@@ -25,6 +25,7 @@
   virtual ~RealSystemState() {}
 
   virtual bool IsOOBEComplete();
+  virtual bool IsOfficialBuild();
 
   virtual inline void set_device_policy(
       const policy::DevicePolicy* device_policy) {