This value was uninitialized before. Initialize it to zero. Also, fix
the tracing output to actually print the start arity and not the end
arity.

R=rossberg@chromium.org

Change-Id: I1eda3be88ca842f60e40e3fb630eca254619ae83
Reviewed-on: https://chromium-review.googlesource.com/756702Reviewed-by: Andreas Rossberg <rossberg@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49176}