If a dependency was unpinned, support adding a revision to it.
R=agable@chromium.org
Bug: 760633
Change-Id: Id2c9fe5174458acaea334726176b88558425ef5a
Reviewed-on: https://chromium-review.googlesource.com/998735
Reviewed-by: Aaron Gable <agable@chromium.org>
Reviewed-by: Michael Moss <mmoss@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>