Add Icelandic language to ChromeOS.

In this change I add Icelandic ("is") to build/config/locales.gni for
ChromeOS only and then update all .grd files to include the language.

Bug: b:175082230
Change-Id: Ib66733855b7ea95bca4dc54474e7970a1a7debf8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2558099
Commit-Queue: Claudio M <claudiomagni@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Finnur Thorarinsson <finnur@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Reviewed-by: Nina Satragno <nsatragno@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Inker Kuo <inker@chromium.org>
Reviewed-by: Angela Yoeurng <yoangela@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Gary Kacmarcik <garykac@chromium.org>
Reviewed-by: Trevor  Perrier <perrier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#857944}