http://unicode.org/cldr/trac/ticket/9943 added a new separate timezone
ID (Etc/UTC) distinct from "Etc/GMT" even though their behavior is
identical. This CLDR change led v8 to violate Ecma 402 requirement that
resolvedOptions().timeZone for DateTimeFormat be "UTC" for "Etc/UTC"
, "Etc/GMT", "GMT" and "UTC".

This CL made v8 compliant to Ecma 402 again.

Bug=v8:6252
TEST=intl/date-format/timezone
Change-Id: Ibe5d3a2a09680ae00de0d73d123a389710c15af0

Change-Id: Ibe5d3a2a09680ae00de0d73d123a389710c15af0
Reviewed-on: https://chromium-review.googlesource.com/496406Reviewed-by: Daniel Ehrenberg <littledan@chromium.org>
Commit-Queue: Jungshik Shin <jshin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45114}