Add a comment explaining why we need libxml's multithreading mode

third_party/libxml/libxml.gyp

 # Copyright (c) 2012 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.
 
 {
   'variables': {
     'conditions': [
       # Define an "os_include" variable that points at the OS-specific generated
       # headers.  These were generated by running the configure script offline.
       ['os_posix == 1 and OS != "mac" and OS != "ios"', {
@@ skipping 200 matching lines @@
           },
           'conditions': [
             ['OS=="linux"', {
               'link_settings': {
                 'libraries': [
                   # We need dl for dlopen() and friends.
                   '-ldl',
                 ],
               },
             }],
+            # http://www.xmlsoft.org/threads.html says that this is required
+            # when using libxml from several threads, which can possibly happen
+            # in chrome. On linux, this is picked up by transitivity from
+            # pkg-config output from build/linux/system.gyp.
             ['OS=="mac" or OS=="android"', {'defines': ['_REENTRANT']}],
             ['OS=="win"', {
               'product_name': 'libxml2',
               # Disable unimportant 'unused variable' warning, and
               # signed/unsigned comparison warning. The signed/unsigned (4101)
               # is fixed upstream and can be removed eventually.
               'msvs_disabled_warnings': [ 4018, 4101 ],
             }, {  # else: OS!="win"
               'product_name': 'xml2',
             }],
@@ skipping 13 matching lines @@
                 '-Wno-pointer-sign',
                 '-Wno-empty-body',
               ],
             }],
           ],
         }],
       ],
     },
   ],
 }