v8 gn build: Support buildling Win snapshots from OS X.

snapshot_toolchain.gni

 # Copyright 2015 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 21 matching lines @@
 # x64 Linux host. Eventually we need to support all of the host/target
 # configurations v8 runs on.
 if (host_cpu == "x64" && host_os == "linux") {
   if (target_cpu == "arm" || target_cpu == "mipsel" || target_cpu == "x86") {
     snapshot_toolchain = "//build/toolchain/linux:clang_x86"
   } else if (target_cpu == "x64" || target_cpu == "arm64" || target_cpu == "mips64el") {
     snapshot_toolchain = "//build/toolchain/linux:clang_x64"
   } else {
     assert(false, "Need environment for this arch: $target_cpu")
   }
+} else if (host_os == "mac" && target_os == "win"){
+  snapshot_toolchain = "//build/toolchain/mac:clang_$target_cpu"

[Nico, 2016/05/11 12:53:10]

We currently only have a mac:clang_x64 toolchain, no mac:clang_x86. We could add
one, but for now I have an assert that target=win host=mac target_cpu=x86 isn't
supported and that folks who want to do cross builds like this (which don't yet
work for other reasons anyhow) should do 64-bit builds.

 } else {
   snapshot_toolchain = default_toolchain
 }