Add node_info.output_for to Event -- a list of node paths whose output
an event represents. For normal nodes it contains just the node own
path. When use_as_output=True delegation is used, it includes the full
ancestor chain (child path + all delegating parent paths).
Also adds a contributing sample demonstrating use_as_output with a
downstream edge in the new Workflow architecture.
Change-Id: I7e11276f894a3a2b396759dd146c221b487d4a7c
Shangjie Chen
d56e11ed68