This patch re-lands #36193 with a couple of minor tweaks
including updating the BUILD.gn file to use the new
variable name and incorporating thakis' request to support
cross-compiling the win snapshot from a mac.

R=thakis@chromium.org, jochen@chromium.org, machenbach@chromium.org
BUG=608596, 595653
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:chromeos_daisy_chromium_compile_only_ng

Review-Url: https://codereview.chromium.org/1979883002
Cr-Commit-Position: refs/heads/master@{#36342}