Update existing uses of /wd4267 to use the GN config

third_party/smhasher/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.
 
 source_set("murmurhash3") {
   sources = [
     "src/MurmurHash3.cpp",
     "src/MurmurHash3.h",
   ]
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
 }
 
 source_set("pmurhash") {
   sources = [
     "src/PMurHash.c",
     "src/PMurHash.h",
   ]
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
 }
 
 source_set("cityhash") {
   sources = [
     "src/City.cpp",
     "src/City.h",
   ]
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
-
-  if (is_win) {
-    # TODO(jschuh): http://code.google.com/p/smhasher/issues/detail?id=19
-    cflags = [ "/wd4267" ]
-  }
 }