Fix Win GN error in mesa's file.

third_party/mesa/BUILD.gn

 # Copyright 2014 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/ui.gni")
 
 config("mesa_headers_config") {
   include_dirs = [ "src/include" ]
   if (use_x11) {
     defines = [ "MESA_EGL_NO_X11_HEADERS" ]
@@ skipping 95 matching lines @@
     }
 
     if (!is_android && !is_mac) {
       cflags += [ "-fPIC" ]
     }
   }
 
   if (is_win) {
     # TODO(scottmg): http://crbug.com/143877 These should be removed if
     # Mesa is ever rolled and the warnings are fixed.
-    cflags = [
+    cflags += [
       "/wd4005",  # Macro defined twice.
       "/wd4065",  # Switch statement contains 'default' but no 'case' labels.
       "/wd4090",  # 'Operation' : different 'modifier' qualifiers
       "/wd4099",  # Type name struct-vs-class doesn't match.
       "/wd4267",  # size_t to type.
       "/wd4273",  # Inconsistent DLL linkage.
       "/wd4291",  # No matching operator delete found for placement new.
       "/wd4305",  # Truncation from int to float.
       "/wd4334",  # Result of 32-bit shift implicitly converted to 64 bits.
       "/wd4345",  # POD-type default initializers.
@@ skipping 563 matching lines @@
       "BUILD_GL32",
       "KEYWORD1=GLAPI",
       "KEYWORD2=GLAPIENTRY",
     ]
   }
 }
 
 }  # !is_android
 
 # TODO(GYP) Android osmesa_in_lib_dir target.