Revert of [gn] Switch win clang bot to gn (patchset #1 id:1 of https://codereview.chromium.org/2310863002/ )

Reason for revert:
Fails compilation:
https://build.chromium.org/p/client.v8/builders/V8%20Win64%20-%20clang

Original issue's description:
> [gn] Switch win clang bot to gn
>
> BUG=chromium:474921
> NOTRY=true
>
> Committed: https://crrev.com/1740587772bbd1ffb7b79a90deb16519e8d47588
> Cr-Commit-Position: refs/heads/master@{#39156}

TBR=jochen@chromium.org,vogelheim@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:474921

Review-Url: https://codereview.chromium.org/2311853002
Cr-Commit-Position: refs/heads/master@{#39158}