Check in the libxml roll script.

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 || is_nacl) {
   os_include = "linux"
 } else if (is_mac || is_ios) {
   os_include = "mac"
@@ skipping 53 matching lines @@
     cflags = [
       # gcc spits out a bunch of warnings about passing too many arguments to
       # __xmlSimpleError.
       "-Wno-format-extra-args",
     ]
   }
 }
 
 static_library("libxml") {
   output_name = "libxml2"
+  # Commented out sources are libxml2 files we do not want to include. They are
+  # here to make it easy to identify files which are new.
   sources = [
     "chromium/libxml_utils.cc",
     "chromium/libxml_utils.h",
     "linux/config.h",
     "linux/include/libxml/xmlversion.h",
     "mac/config.h",
     "mac/include/libxml/xmlversion.h",
     #"src/DOCBparser.c",
     "src/HTMLparser.c",
     "src/HTMLtree.c",
@@ skipping 71 matching lines @@
     "src/save.h",
     #"src/schematron.c",
     "src/threads.c",
     "src/timsort.h",
     "src/tree.c",
     "src/triodef.h",
     "src/trionan.h",
     #"src/trio.c",
     #"src/trio.h",
     #"src/triodef.h",
+    # Note: xpath.c #includes trionan.c
     #"src/trionan.c",
-    #"src/trionan.h",
     #"src/triop.h",
     #"src/triostr.c",
     #"src/triostr.h",
     "src/uri.c",
     "src/valid.c",
     #"src/xinclude.c",
     #"src/xlink.c",
     "src/xmlIO.c",
     "src/xmlmemory.c",
     #"src/xmlmodule.c",
@@ skipping 32 matching lines @@
   if (is_mac || is_ios || is_android) {
     # 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.
     defines = [ "_REENTRANT" ]
   }
 
   include_dirs = [ "$os_include" ]
 }