With certain ICU data bundles (such as the Node.js "small-icu"),
%GetDefaultICULocale() may return a more specific language tag (e.g.
"en-US") than what's available (e.g. "en"). In those cases, consider the
more specific language tag supported.

This CL also resolves the following Node.js issue:
   https://github.com/nodejs/node/issues/15223

Bug: v8:7024
Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: Ifda0776b3418734d5caa8af4e50c17cda95add73
Reviewed-on: https://chromium-review.googlesource.com/668350
Commit-Queue: Daniel Ehrenberg <littledan@chromium.org>
Reviewed-by: Daniel Ehrenberg <littledan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52716}