Revert of Change v8_snapshot_toolchain to be a GN build arg. (patchset #3 id:40001 of https://codereview.chromium.org/1969693002/ )

Reason for revert:
Blocks the roll: https://codereview.chromium.org/1974733002/

Breaks:
https://build.chromium.org/p/tryserver.chromium.linux/builders/chromeos_daisy_chromium_compile_only_ng/builds/136166

Original issue's description:
> Change v8_snapshot_toolchain to be a GN build arg.
>
> It is likely that CrOS and other distros will want to be
> able to build the v8 snapshot with a custom toolchain.
>
> This CL makes snapshot_toolchain a declared build arg, and
> renames it to v8_snapshot_toolchain to minimize the risk
> of a name collision.
>
> R=machenbach@chromium.org, jochen@chromium.org
> BUG=608596
>
> Committed: https://crrev.com/2ae74af8f405f461d8e44d60b978f940673e46c8
> Cr-Commit-Position: refs/heads/master@{#36193}

TBR=jochen@chromium.org,dpranke@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=608596

Review-Url: https://codereview.chromium.org/1975713002
Cr-Commit-Position: refs/heads/master@{#36199}