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

third_party/libxml/libxml.gyp

Index: third_party/libxml/libxml.gyp
diff --git a/third_party/libxml/libxml.gyp b/third_party/libxml/libxml.gyp
index 79a1bc23a79f1c2d4efe7e6b54b9d8d266551b9a..a180ee6e671d0b18b81e796d42d5e0b32a79a220 100644
--- a/third_party/libxml/libxml.gyp
+++ b/third_party/libxml/libxml.gyp
@@ -218,6 +218,10 @@
                 ],
               },
             }],
+            # 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',