SOCKS v5 implementation

      The implementation is incomplete as it does not support any authentication methods and does not have a UDP layer.

BUG=469
TEST=unittests (included)

Review URL: http://codereview.chromium.org/150187

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@20724 0039d316-1c4b-4281-b951-d872f2087c98