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Prior to this CL, Chrome on desktop Linux often inconsistently used
contour fonts (e.g., DejaVu Sans and Noto Sans Symbols2) for some
emojis, even when a dedicated emoji font (e.g., Noto Color Emoji) is
available and used for all other emojis. In order to force the color
emoji font to be used always, hacks [1] have been mentioned in online
forums that prepend the color emoji font to the list of system fonts in
fonts.conf, which can have highly deletorious effects on other
applications (like normal numerals becoming their emoji versions).

The ISO locale code "und-Zsye" [2] is one of the ways of informing
fontconfig (as of version 2.12.5) that color emoji fonts should be
preferred, along with the special font family "emoji" [3]. fontconfig
also has default configurations that automatically map the locale and
the "emoji" family to any available color emoji font. By explicitly
setting the locale to "und-Zsye", this CL provides the desired
out-of-the-box experience to desktop Linux users.

While we could also specify the "emoji" font family either instead of or
in addition to the locale, doing so requires more plumbing in the code
and may be done in the future.

[1]: https://github.com/googlefonts/noto-emoji/issues/36
[2]: https://unicode.org/reports/tr51/#Emoji_Script
[3]: https://bugs.freedesktop.org/show_bug.cgi?id=94551

Bug: 767754
Change-Id: Ie8019cc0540a177c816c8131ca7c6a504952aa09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2207627
Commit-Queue: Timothy Gu <timothygu@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772414}