libxml: move all upstream code into a src/ subdirectory.
This helps keep our patches, build files, and metadata separate.
(Note that src/ is not purely what upstream has, since it has our
patches applied.)
Review URL: http://codereview.chromium.org/3160012
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@56107 0039d316-1c4b-4281-b951-d872f2087c98