Move HttpAuth::ChallengeTokenizer to its own file.

Avoids a circulate dependency in preparation of using this tokenizer to strip
private data from net-export logs.

BUG=349502

Review URL: https://codereview.chromium.org/191563004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256715 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256715