GN/Win: support win_toolchain asset?

CQ_INCLUDE_TRYBOTS=master.client.skia.compile:Build-Win-MSVC-x86_64-Debug-GN-Trybot

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3246

Change-Id: Ib38d4f16788bc27ad81975a4c1b0a732e5df1de3
Reviewed-on: https://skia-review.googlesource.com/3246
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
4 files changed