Move the language code checking for 4 languages requiring
special case mapping to C++ from JavaScript.

This is a speculative fix for crashes reported from Windows and
Mac Chrome canary builds when icu-case-mapping is enabled by
default. (see crbug.com/676643)

In addition, tighten up comparision operators in a couple of
places in i18n.js (=== and !== instead of == and !=).

BUG=v8:4477, v8:4476, chromium:676643
TEST=test262/{built-ins,intl402}/Strings/*, webkit/fast/js/*,
     mjsunit/string-case, intl/general/case*

Review-Url: https://codereview.chromium.org/2621393002
Cr-Commit-Position: refs/heads/master@{#42246}