i18n: Fix new Intl.DateTimeFormat("ja-u-tz-uslax")

Fix the locale parsing with tz which resolved to a tz name which
have a _ in it. "uslax" is resolved to "Los_Angles" and cause that.

TEST=In v8 or Chrome console, new Intl.DateTimeFormat("ja-u-tz-uslax") works fine.
resolvedOptions().timeZone is 'America/Los_Angenles'.
Bug: v8:8469
Change-Id: I996790b1ce59fbb5f6b0a460ff8acdb674894df1
Reviewed-on: https://chromium-review.googlesource.com/c/1372810
Reviewed-by: Jungshik Shin <jshin@chromium.org>
3 files changed