Land prep work to enable NaCl in the Linux x64 GN builds.

third_party/libxml/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.
 
 # Define an "os_include" variable that points at the OS-specific generated
 # headers.  These were generated by running the configure script offline.
-if (is_linux || is_android) {
+if (is_linux || is_android || is_nacl) {
   os_include = "linux"
 } else if (is_mac || is_ios) {
   os_include = "mac"
 } else if (is_win) {
   os_include = "win32"
 }
 
 config("libxml_config") {
   # Define LIBXML_STATIC as nothing to match how libxml.h (an internal header)
   # defines LIBXML_STATIC, otherwise we get the macro redefined warning from
@@ skipping 157 matching lines @@
       "-Wno-tautological-pointer-compare",
 
       # threads.c attempts to forward declare a pthread_equal which doesn't
       # match the prototype in pthreads.h
       "-Wno-ignored-attributes",
     ]
   }
 
   include_dirs = [ "$os_include" ]
 }