[spellcheck] Add cy-GB (Welsh) to list of dictionaries

Add initial Welsh dictionary to spellcheck list.

Bug: 692242
Change-Id: Ic9f0d030170ef62af2aa10ba422585659c7c7359
Reviewed-on: https://chromium-review.googlesource.com/c/1477341
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Commit-Queue: Kevin Bailey <krb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635521}
1 file changed