Add identifiers for rangeOverflow/rangeUnderflow localized strings.

We show a rangeOverflow message for input[type=date] like
    "Value must be less than or equal to 09/30/2013."
"greater than" is not appropriate for dates. We'd like to show something like
    "Value must be 09/30/2013 or earlier."

This CL is a preparation to implement it.

BUG=161733
R=tony@chromium.org

Review URL: https://codereview.chromium.org/24051006

git-svn-id: svn://svn.chromium.org/blink/trunk@158063 bbb929c8-8fbe-4397-9dbb-9b2b20218538