Add a UseCounter for default in custom-ident

The default keyword is reserved and is also not a valid <custom-ident>.[1]
We should deprecate and remove support for default in custom-ident.
Add a UseCounter for default in custom-ident.

[1]: https://drafts.csswg.org/css-values-4/#custom-idents

Bug: 882285
Change-Id: I1c102200a641c6a2a1266a95a682e367f28b6d5c
Reviewed-on: https://chromium-review.googlesource.com/c/1325599
Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Anders Ruud <andruud@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607809}
27 files changed