Do not create multiple Locale objects for the default locale.

Remove Locale::createDefault, and introduce Local::defaultLocale, which return a
static Locale object for defaultLanguage().

BUG=298020
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/25194002

git-svn-id: svn://svn.chromium.org/blink/trunk@158507 bbb929c8-8fbe-4397-9dbb-9b2b20218538