Revert of Move (hopefully) remaining isolate related variables to toolchain.gypi (patchset #1 id:1 of https://codereview.chromium.org/2027213002/ )

Reason for revert:
Fails:
https://build.chromium.org/p/client.v8.fyi/builders/V8-Blink%20Linux%2064/builds/7182/steps/gclient%20runhooks/logs/stdio

Original issue's description:
> Move (hopefully) remaining isolate related variables to toolchain.gypi
>
> BUG=chromium:609107
> R=machenbach@chromium.org
>
> Committed: https://crrev.com/a68a1eb38dda0a1eba3b7644a172d2f85e9c7abe
> Cr-Commit-Position: refs/heads/master@{#36664}

TBR=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/2038463002
Cr-Commit-Position: refs/heads/master@{#36665}