Don't allow rollback on the stable-channel.

This change explicitly disallows rollback on the stable-channel.

BUG=chromium:252539
TEST=unittests + on device.

Change-Id: I71caf95daae185790cf9fb0fda790eaf960bbe6b
Reviewed-on: https://gerrit.chromium.org/gerrit/61687
Tested-by: Chris Sosa <sosa@chromium.org>
Reviewed-by: David Zeuthen <zeuthen@chromium.org>
Commit-Queue: Chris Sosa <sosa@chromium.org>
diff --git a/omaha_request_params.h b/omaha_request_params.h
index 7a85500..1a65c75 100644
--- a/omaha_request_params.h
+++ b/omaha_request_params.h
@@ -216,6 +216,9 @@
 
   // For unit-tests.
   void set_root(const std::string& root);
+  void set_current_channel(const std::string& channel) {
+    current_channel_ = channel;
+  }
 
   // Enforce security mode for testing purposes.
   void SetLockDown(bool lock);