Check in libxml in third_party/

we're trying to move away from using the gitdeps.py script and libxml is
the only third_party dependency that doesn't have a git mirror for it,
as we're mostly in maintenance mode now there's no real downside in
just checking it in in our repo.

TBR: chrisha@chromium.org
Bug: 759092
Change-Id: If78a56bc2aedd5fe583359ae4c41258c7db00455
Reviewed-on: https://chromium-review.googlesource.com/809233
Reviewed-by: S├ębastien Marchand <sebmarchand@chromium.org>
222 files changed