Pass shell name instead of an absolute path.

Bug: v8:796166
Change-Id: Ia9472e893fd2cb3fde2a94997f3e9daf30da06ea
Reviewed-on: https://chromium-review.googlesource.com/833917
Commit-Queue: Michał Majewski <majeski@google.com>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50207}