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

src/trusted/win/BUILD.gn

 # Copyright (c) 2015 The Native Client Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/toolchain/toolchain.gni")
 
 # This is a special toolchain used solely for building GNU/AT&T-syntax
 # assembly files on Windows.  See src/trusted/asm.gni for more details.
 
 template("nacl_win_as_toolchain") {
@@ skipping 41 matching lines @@
     }
 
     toolchain_args() {
       current_os = "win"
       current_cpu = invoker.toolchain_cpu
 
       # These values need to be passed through unchanged.
       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
 }