Add ValidationTooShort to localized sring header in preparation for a message displayed when a form input content is shorter than minlength attribute.

BUG=390485

Review URL: https://codereview.chromium.org/632623002

git-svn-id: svn://svn.chromium.org/blink/trunk@183258 bbb929c8-8fbe-4397-9dbb-9b2b20218538