Basically:
  * user preps upstream change + downstream changes
  * user links downstream changes into description of upstream change, e.g.:
      DOWNSTREAM_CL=project_name=https://chromiumcodereview.appspot.com/....
      DOWNSTREAM_CL=...
  * lgtm's all around
  * user commits upstream change
  * roller picks up upstream change, and tries to apply it to 'project_name' downstream repo. When attempting to apply it, it will upload the recipes.cfg change to the indicated CL, and apply the indicated CL before running simulation. If the simulation is clean and the downstream CL is lgtm'd, it CQ's that downstream CL (which, itself, might have DOWNSTREAM_CL links)

Additionally, we should have tools to help the user prepare all of these linked CLs.