Revert of Remove v8_toolset_for_d8 flag (patchset #1 id:1 of https://codereview.chromium.org/1969793002/ )

Reason for revert:
blocks roll: https://codereview.chromium.org/1975753002/

Original issue's description:
> Remove v8_toolset_for_d8 flag
>
> R=machenbach@chromium.org
> BUG=chromium:609107
>
> Committed: https://crrev.com/6cb38427984f3c31ff6c8813535cfd6ebf6058c4
> Cr-Commit-Position: refs/heads/master@{#36198}

TBR=machenbach@chromium.org,jochen@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:609107

Review-Url: https://codereview.chromium.org/1974773002
Cr-Commit-Position: refs/heads/master@{#36209}