It fails when both the revision and the url are None.

Bug: 841936
Change-Id: Idef45a015624a92226d4ccd38ed5b978bf786993
Reviewed-on: https://chromium-review.googlesource.com/1053996
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Ryan Tseng <hinoka@chromium.org>