manually regen distribution.proto and backend.proto (#148)

See b/123287260. Currently backend.proto and distribution.proto live in
both googleapis/googleapis and googleapis/api-common-protos. The less
up-to-date one is being used (api-common-protos). This commit is created
by pulling down api-common-protos, deleting backend.proto and
distribution.proto, and running the generation scripts with COMMONPROTOS
set to the path of the modified api-commons-proto.