Add Syzygy support to Windows GN build.

build/config/sanitizers/sanitizers.gni

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 declare_args() {
   # Use libc++ (buildtools/third_party/libc++ and
   # buildtools/third_party/libc++abi) instead of stdlibc++ as standard library.
   # This is intended to be used for instrumented builds.
   use_custom_libcxx = (is_asan && is_linux) || is_tsan || is_msan
 
   # Track where uninitialized memory originates from. From fastest to slowest:
   # 0 - no tracking, 1 - track only the initial allocation site, 2 - track the
   # chain of stores leading from allocation site to use site.
   msan_track_origins = 2
 
   # Use dynamic libraries instrumented by one of the sanitizers instead of the
   # standard system libraries. Set this flag to download prebuilt binaries from
   # GCS.
   use_prebuilt_instrumented_libraries = false
+
+  # Enable building with SyzyAsan which can find certain types of memory
+  # errors. Only works on Windows. See
+  # https://code.google.com/p/sawbuck/wiki/SyzyASanHowTo
+  is_syzyasan = false
 }