.. for side effect free debug evaluate.

R=yangguo@chromium.org

Bug: v8:7588
Change-Id: Iac4d782dbf996d9c11430fc681f38a648d89435b
Reviewed-on: https://chromium-review.googlesource.com/1000527
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52699}