Fix GN build on Linux.

tools/gn/secondary/third_party/WebKit/Source/wtf/BUILD.gn

 # Copyright 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.
 
 config("wtf-config") {
   include_dirs = [ ".." ]
   if (is_win) {
     include_dirs += [ "os-win32" ]
     defines = [
       "__STD_C",
       "_CRT_SECURE_NO_DEPRECATE",
       "_SCL_SECURE_NO_DEPRECATE",
       "CRASH=__debugbreak",
     ]
 #    # Chromium windows multi-dll build enables c++ exception and this
 #    # causes wtf generates 4291 warning due to operator new/delete
 #    # implementations. Disable the warning for chromium windows
 #    # multi-dll build.
 #    'msvs_disabled_warnings': [4291],
 #    'direct_dependent_settings': {
 #      'msvs_disabled_warnings': [4291],
 #    },
   }
 
   if (is_linux) { # (gcc_version >= 46) {
     # Disable warnings about c++0x compatibility, as some names (such as
     # nullptr) conflict with upcoming c++0x types.
-    cflags_cc += [ "-Wno-c++0x-compat" ]
+    cflags_cc = [ "-Wno-c++0x-compat" ]
   }
 
 #  # Some warnings occur in WTF headers, so they must also be disabled
 #  # in targets that use WTF.
 #  'msvs_disabled_warnings': [
 #    # Don't complain about calling specific versions of templatized
 #    # functions (e.g. in RefPtrHashMap.h).
 #    4344,
 #    # Don't complain about using "this" in an initializer list
 #    # (e.g. in StringImpl.h).
@@ skipping 319 matching lines @@
   #  # Disable c4267 warnings until we fix size_t to int truncations.
 # 'msvs_disabled_warnings': [4127, 4355, 4510, 4512, 4610, 4706, 4068, 4267],
 
 #  if (is_linux && use_tcmalloc) {
 #     deps += [
 #      "//base:base",
 #      "//base/allocator:allocator",
 #    ],
 #  }
 }