Check in the libxml roll script.

third_party/libxml/BUILD.gn

Index: third_party/libxml/BUILD.gn
diff --git a/third_party/libxml/BUILD.gn b/third_party/libxml/BUILD.gn
index 41c0fa716c4b44aa49cf0ad61aa123fa1c43d215..28ac4d034e65e6285dba25e8ceedb2f3a816bd9c 100644
--- a/third_party/libxml/BUILD.gn
+++ b/third_party/libxml/BUILD.gn
@@ -71,6 +71,8 @@ config("libxml_warnings") {
 
 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",
@@ -162,8 +164,8 @@ static_library("libxml") {
     #"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",