Custom deps not present in DEPS files cause errors when syncing, since
we add them as strings in postprocess_deps, but deps_to_objects expects
a dictionary.

TBR=agable@chromium.org

Bug: 839925
Change-Id: Ic08a83e8692f1bf90d4456c72fe99493363ba747
Reviewed-on: https://chromium-review.googlesource.com/1063326
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>