Fix Wvarargs warning in XFA error code.

build_gyp/standalone.gypi

 # Copyright 2014 PDFium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Definitions to be used when building stand-alone PDFium binaries.
 
 {
   'variables': {
     'component%': 'static_library',
     'clang%': 0,
@@ skipping 376 matching lines @@
         '-fno-strict-aliasing',
       ],
       'WARNING_CFLAGS': [
         '-Wall',
         '-Wendif-labels',
         '-W',
         '-Wno-unused-parameter',
       ],
     },
     'variables': {
-      'clang_warning_flags': [],
+      'clang_warning_flags': [
+        # TODO(thakis): https://crbug.com/604888
+        '-Wno-undefined-var-template',
+      ],
     },
     'includes': [ 'set_clang_warning_flags.gypi', ],
     'conditions': [
       ['component=="shared_library"', {
         'cflags': [
           '-fPIC',
         ],
       }],
       ['asan==1', {
         'defines': [
@@ skipping 90 matching lines @@
       ],
     }],  # OS=="win"
     ["use_goma==1", {
       'make_global_settings': [
         ['CC_wrapper', '<(gomadir)/gomacc'],
         ['CXX_wrapper', '<(gomadir)/gomacc'],
       ],
     }],  # use_goma==1
   ],
 }