- remove unused Runtime_GetLanguageTagVariants
- add test for another related bug (chromium:770452) as well as for 
chromium:770450 . 

Bug: chromium:770450, chromium:770452
Test: intl/general/invalid-locale.js
Cq-Include-Trybots: master.tryserver.v8:v8_linux_noi18n_rel_ng
Change-Id: I4496a4a5421000faa0e37aed85fea21ceb487998
Reviewed-on: https://chromium-review.googlesource.com/710816Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Jungshik Shin <jshin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48483}