Add Syzygy support to Windows GN build.

build/config/BUILD.gn

Index: build/config/BUILD.gn
diff --git a/build/config/BUILD.gn b/build/config/BUILD.gn
index 287a1e762b52e84c1d114dca794af45e6a353e6c..56a86e5932783d772e57bf1885f42cbe3ce961c0 100644
--- a/build/config/BUILD.gn
+++ b/build/config/BUILD.gn
@@ -6,6 +6,7 @@ import("//build/config/allocator.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
+import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/ui.gni")
 import("//build/module_args/v8.gni")
 
@@ -275,6 +276,13 @@ config("feature_flags") {
   if (enable_webvr) {
     defines += [ "ENABLE_WEBVR" ]
   }
+  if (is_syzyasan) {
+    defines += [
+      "SYZYASAN",
+      "MEMORY_TOOL_REPLACES_ALLOCATOR",
+      "MEMORY_SANITIZER_INITIAL_SIZE",
+    ]
+  }
 }
 
 # Debug/release ----------------------------------------------------------------