Bug 1733672 [wpt PR 31065] - Add viewport segments CSS environment variables, a=testonly

Bug 1733672 [wpt PR 31065] - Add viewport segments CSS environment variables, a=testonly

Automatic update from web-platform-tests
Add viewport segments CSS environment variables

The viewport-segment-* set of environment variables are two dimensional,
i.e. you need an x and y values in order to access them. This change
adds support for such environment variables to CSS parser, style
resolution and the StyleEnvironmentVariables class. The variable values
are stored separately in a two dimensional vector, and new methods that
explicitly operate on two dimensional variables (via an explicit enum)
have been introduced.

Additionally, this CL converts the previous fold-* variables to the new
viewport-segment variables with the indexed syntax.

Bug: 1039051
Change-Id: Id520ee47f4c9bd79bc47ee1693c80750f2a8779b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3195791
Commit-Queue: Daniel Libby <dlibby@microsoft.com>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Auto-Submit: Daniel Libby <dlibby@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#928050}

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wpt-commits: 25db855bf307d50924dc30cd2557784930d75248
wpt-pr: 31065