[Chrome OS] Refactor Canonicalize() method into base class to prepare for new Authenticator subclass

BUG=chromium-os:4929
TEST=unit tests

Review URL: http://codereview.chromium.org/3436031

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@60752 0039d316-1c4b-4281-b951-d872f2087c98