This removes 1 RPC when tbr-ing second or later patchset.

Since tests cover TBR only in first upload, there are
no test changes. Since adding test is real PITA here,
keep them as is.

R=ehmaldonado

Change-Id: I90801b06727ce2131a703d6cd6c24ffad798bc8a
Reviewed-on: https://chromium-review.googlesource.com/1242212Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>