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/omaha_response_handler_action_unittest.cc b/omaha_response_handler_action_unittest.cc
index 15a1f06..ae8fccb 100644
--- a/omaha_response_handler_action_unittest.cc
+++ b/omaha_response_handler_action_unittest.cc
@@ -77,6 +77,11 @@
                 SetString(kPrefsUpdateCheckResponseHash, in.hash))
         .WillOnce(Return(true));
   }
+
+  string current_url = in.payload_urls.size() ? in.payload_urls[0] : "";
+  EXPECT_CALL(*(mock_system_state->mock_payload_state()), GetCurrentUrl())
+      .WillRepeatedly(Return(current_url));
+
   OmahaResponseHandlerAction response_handler_action(mock_system_state);
   response_handler_action.set_boot_device(boot_dev);
   BondActions(&feeder_action, &response_handler_action);