Revert "[build] Remove unused toolchain update script"

This reverts commit c6c6cdb9.

Reason for revert: Missed this:
https://cs.chromium.org/chromium/src/v8/gypfiles/gyp_environment.py?q=gypfiles/gyp_environment&sq=package:chromium&l=13

Original change's description:
> [build] Remove unused toolchain update script
> 
> TBR=sergiyb@chromium.org
> 
> Bug: v8:6857
> Change-Id: I2b02d3352e5b13b73ed74a76405e15c266593873
> Reviewed-on: https://chromium-review.googlesource.com/690079
> Commit-Queue: Michael Achenbach <machenbach@chromium.org>
> Reviewed-by: Michael Achenbach <machenbach@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#48216}

TBR=machenbach@chromium.org,sergiyb@chromium.org

Change-Id: I255161b348ac2732cb350ebf8039050ca864f857
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:6857
Reviewed-on: https://chromium-review.googlesource.com/691835Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48217}