presubmit: Make AUTHORS presubmit check use lower-cased email addresses.

This is so that presubmit stops complaining about Takano.Naoki@gmail.com.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/7024048

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@88004 0039d316-1c4b-4281-b951-d872f2087c98