Unbreak clang/win tot bots after clang r302190

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index fa983d6aea48068c378d6e24fb2ed89c8f662e80..b6483fa216eb63891f3e7790cba885b6c4c4bc58 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -1018,6 +1018,9 @@ config("default_warnings") {
         "-Wno-unknown-pragmas",  # http://crbug.com/505314
         "-Wno-microsoft-cast",  # http://crbug.com/550065
       ]
+      if (llvm_force_head_revision) {
+        cflags += [ "-Wno-microsoft-enum-forward-reference" ]  # http://crbug.com/718880
+      }
     }
   } else {
     if (is_mac && !is_nacl) {