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

snapshot_toolchain.gni

Index: snapshot_toolchain.gni
diff --git a/snapshot_toolchain.gni b/snapshot_toolchain.gni
index 4932110489dd1a7fe65a6124aa655192661d3bd6..5831716e3bed1b6192e45b514b7021d51b65bdcf 100644
--- a/snapshot_toolchain.gni
+++ b/snapshot_toolchain.gni
@@ -39,6 +39,8 @@ if (host_cpu == "x64" && host_os == "linux") {
   } 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
 }