If a command prefix should be used in predictable mode, it should
wrap around d8 and not around the predictable wrapper.

NOTRY=true
TBR=majeski@google.com

Bug: v8:7166
Change-Id: I16f33090c647ea4f6f040c6194908df6500b82a3
Reviewed-on: https://chromium-review.googlesource.com/832206
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50160}