Add isolate_win_toolchain.py

Intended to be run manually by a developer on Windows.
Downloads the requested Windows toolchain, uploads it
to the isolate server, records the isolated hash in
the JSON file and uploads a CL to update that file.

BUG=skia:4553, skia:4763
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1775073002

Review URL: https://codereview.chromium.org/1775073002
4 files changed