Upgrade to libxml 2.9.2 and libxslt 1.1.28

      The previous version of libxml was released in 2008, so this is a large
jump.

One notable functionality change is that the parser no longer accepts

  <stuff xmlns:stream=&quota&quotxmlns=&quotb&quot>

(with no space between the end quote of &quota&quot and the xmlns= attribute).

This seems correct to not accept, but could potentially cause some minor
compatibility differences. This is the change in
xmpp_login_handler_unittest.js.

A second difference is that the column number reported in error
conditions has changed in some cases. This causes some expected-error
LayoutTests to differ in textual output. These seem reasonable and
should hopefully not cause any major compatibility issues.

Blink suppressions at https://codereview.chromium.org/1181403003/ which
need to land first.

BUG=463958,502468

Review URL: https://codereview.chromium.org/1193533007

Cr-Commit-Position: refs/heads/master@{#335721}