Instead of unconditionally copying the script name into a
heap-allocated char vector, use existing storage if possible. Also, try
to avoid materializing the script name for computing its length.

R=titzer@chromium.org

Change-Id: If0e8ac95ecbbb4e7463d9e4a4cdb5579270bcdaf
Reviewed-on: https://chromium-review.googlesource.com/1051230Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53157}