Roll libxml to 3939178e4cb797417ff033b1e04ab4b038e224d9

Removes a few patches fixed upstream:
https://git.gnome.org/browse/libxml2/commit/?id=e26630548e7d138d2c560844c43820b6767251e3
https://git.gnome.org/browse/libxml2/commit/?id=94691dc884d1a8ada39f073408b4bb92fe7fe882

Stops using the NOXXE flag which was reverted upstream:
https://git.gnome.org/browse/libxml2/commit/?id=030b1f7a27c22f9237eddca49ec5e620b6258d7d

Changes the patch to uri.c to not add limits.h, which is included
upstream.

Bug: 722079
Change-Id: I4b8449ed33f95de23c54c2cde99970c2df2781ac
Reviewed-on: https://chromium-review.googlesource.com/535233
Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Dominic Cooney <dominicc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#480755}
 bool XmlReader::LoadFile(const std::string& file_path) {
   const int kParseOptions = XML_PARSE_RECOVER |  // recover on errors
                            XML_PARSE_NONET;     // forbid network access
   reader_ = xmlReaderForFile(file_path.c_str(), NULL, kParseOptions);
   return reader_ != NULL;
 }

 bool XmlReader::LoadFile(const std::string& file_path) {
   const int kParseOptions = XML_PARSE_RECOVER |  // recover on errors
                            XML_PARSE_NONET |    // forbid network access
                            XML_PARSE_NOXXE;     // no external entities
   reader_ = xmlReaderForFile(file_path.c_str(), NULL, kParseOptions);
   return reader_ != NULL;
 }
