Turn off ASAN for non-default toolchains for win clang.

src/trusted/win/BUILD.gn

Index: src/trusted/win/BUILD.gn
diff --git a/src/trusted/win/BUILD.gn b/src/trusted/win/BUILD.gn
index cd52e3954e2f96643f680717a9cbf5f65d5b243d..e8754751f80016ae728be777fed85351a470fdec 100644
--- a/src/trusted/win/BUILD.gn
+++ b/src/trusted/win/BUILD.gn
@@ -59,14 +59,20 @@ template("nacl_win_as_toolchain") {
       host_toolchain = host_toolchain
       target_os = target_os
       target_cpu = target_cpu
+
+      if (defined(invoker.is_asan)) {
+        is_asan = invoker.is_asan
+      }
     }
   }
 }
 
 nacl_win_as_toolchain("nacl_win_as_x86") {
   toolchain_cpu = "x86"
+  is_asan = false
 }
 
 nacl_win_as_toolchain("nacl_win_as_x64") {
   toolchain_cpu = "x64"
+  is_asan = false
 }