Etc/GMT* time zones are  listed in the INAN time zone database and
they should be accepted as valid.

This CL will be followed by a CL for moving time zone name checks to C++
that will accept all the time zone names (e.g. EST5EDT, Hongkong, ROK, Zulu).

Bug: chromium:364374
Test: intl/date-format/timezone.js
Test: mjsunit/regress/regress-crbug-364374
Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: If0e5327d7e980504a9cb3d2b641e907ebce61180
Reviewed-on: https://chromium-review.googlesource.com/1159546
Commit-Queue: Jungshik Shin <jshin@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54879}