Script::SetEvalOrigin() could do a stackwalk depending on whether one of
its arguments is a sentinel value. This is somewhat surprising behavior,
and since this function has just one caller, it makes more sense to do
this in that location.

R=mstarzinger@chromium.org

Bug: 
Change-Id: Iddd38b42d9a3a8f4898aafb20fe165dfee51d516
Reviewed-on: https://chromium-review.googlesource.com/643386Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47715}