clang/win: Make builds work with MSVC2015 update 3.

build/config/win/BUILD.gn

 # Copyright (c) 2013 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.
 
 import("//build/config/clang/clang.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/win/visual_studio_version.gni")
 import("//build/toolchain/toolchain.gni")
 
@@ skipping 384 matching lines @@
     "/ZW",
     "/EHsc",
   ]
 }
 
 # Internal stuff --------------------------------------------------------------
 
 # Config used by the MIDL template to disable warnings.
 config("midl_warnings") {
   if (is_clang) {
-    # MIDL generates code like "#endif !_MIDL_USE_GUIDDEF_".
-    cflags = [ "-Wno-extra-tokens" ]
+    cflags = [
+      # MIDL generates code like "#endif !_MIDL_USE_GUIDDEF_".
+      "-Wno-extra-tokens",
+
+      # TODO(thakis): Remove this once clang is rolled past r279116,
+      # https://crbug.com/637456
+      "-Wno-extern-initializer",
+    ]
   }
 }