Language.{h,cpp} clean-up.

- Remove userPreferredLanguagesOverride().  It's unused.
- Rename canonicalizedLanguageIdentifier() to canonicalizeLanguageIdentifier().
- Make sure that platformLanguage() and userPreferredLanguages() return language
  codes with '-'.
- Optimize platformLanguage().  It shouldn't create a Vector every time.

BUG=

Review URL: https://codereview.chromium.org/703043002

git-svn-id: svn://svn.chromium.org/blink/trunk@184894 bbb929c8-8fbe-4397-9dbb-9b2b20218538