One possible implementation:
https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1002892

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/depot_tools/+/54a81e45ddfc976a037e01ca71c9f02d614c1f43

commit 54a81e45ddfc976a037e01ca71c9f02d614c1f43
Author: Kimmo Kinnunen <kkinnunen@nvidia.com>
Date: Fri Apr 13 09:30:30 2018

Add support for downloading Windows VS toolchain from a HTTP URL

Add support for downloading Windows VS toolchain from a HTTP URL.
This is useful for developers that do not have access to the
Chrome toolchain.

If the developer specifies DEPOT_TOOLS_WIN_TOOLCHAIN_HTTP_BASE_URL
environment variable, the toolchain will be downloaded from that
url.

Bug:  830569 
Change-Id: I41d815a4460085c3b028f56f2e01adc68d8410fe
Reviewed-on: https://chromium-review.googlesource.com/1002892
Commit-Queue: Kimmo Kinnunen FI <kkinnunen@nvidia.com>
Reviewed-by: Scott Graham <scottmg@chromium.org>

[modify] https://crrev.com/54a81e45ddfc976a037e01ca71c9f02d614c1f43/win_toolchain/get_toolchain_if_necessary.py

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/aa48bca29fcbecfc21a9ed79a6acb350eff6334c

commit aa48bca29fcbecfc21a9ed79a6acb350eff6334c
Author: depot-tools-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com <depot-tools-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Date: Fri Apr 13 13:34:04 2018

Roll src/third_party/depot_tools/ 53a115e67..54a81e45d (1 commit)

https://chromium.googlesource.com/chromium/tools/depot_tools.git/+log/53a115e67e50..54a81e45ddfc

$ git log 53a115e67..54a81e45d --date=short --no-merges --format='%ad %ae %s'
2018-04-12 kkinnunen Add support for downloading Windows VS toolchain from a HTTP URL

Created with:
  roll-dep src/third_party/depot_tools
BUG= chromium:830569 


The AutoRoll server is located here: https://depot-tools-chromium-roll.skia.org

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff, who should
be CC'd on the roll, and stop the roller if necessary.


TBR=agable@chromium.org

Change-Id: I891b5ae2d18128cdd16a03343ea05136c498974c
Reviewed-on: https://chromium-review.googlesource.com/1011666
Reviewed-by: depot-tools-chromium-autoroll <depot-tools-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Commit-Queue: depot-tools-chromium-autoroll <depot-tools-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#550609}
[modify] https://crrev.com/aa48bca29fcbecfc21a9ed79a6acb350eff6334c/DEPS

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/aa48bca29fcbecfc21a9ed79a6acb350eff6334c

commit aa48bca29fcbecfc21a9ed79a6acb350eff6334c
Author: depot-tools-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com <depot-tools-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Date: Fri Apr 13 13:34:04 2018

Roll src/third_party/depot_tools/ 53a115e67..54a81e45d (1 commit)

https://chromium.googlesource.com/chromium/tools/depot_tools.git/+log/53a115e67e50..54a81e45ddfc

$ git log 53a115e67..54a81e45d --date=short --no-merges --format='%ad %ae %s'
2018-04-12 kkinnunen Add support for downloading Windows VS toolchain from a HTTP URL

Created with:
  roll-dep src/third_party/depot_tools
BUG= chromium:830569 


The AutoRoll server is located here: https://depot-tools-chromium-roll.skia.org

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff, who should
be CC'd on the roll, and stop the roller if necessary.


TBR=agable@chromium.org

Change-Id: I891b5ae2d18128cdd16a03343ea05136c498974c
Reviewed-on: https://chromium-review.googlesource.com/1011666
Reviewed-by: depot-tools-chromium-autoroll <depot-tools-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Commit-Queue: depot-tools-chromium-autoroll <depot-tools-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#550609}
[modify] https://crrev.com/aa48bca29fcbecfc21a9ed79a6acb350eff6334c/DEPS