Bug 1686544 [wpt PR 27158] - [GridNG] Apply justify-content, alignment-content.,...

Bug 1686544 [wpt PR 27158] - [GridNG] Apply justify-content, alignment-content., a=testonly

Automatic update from web-platform-tests
[GridNG] Apply justify-content, alignment-content.

As above. This patch implements the core content alignment logic. E.g.
determining the offset at which the tracks should start, and the space
between them (gutter-size - this include the grid-gap if any).

Creates a new struct "TrackAlignmentGeometry" which contains these
two values. Effectively the process is now:
GridGap -> TrackAlignmentGeometry -> SetOffsets.

The most significant difference is that TrackAlignmentGeometry includes
the grid-gap, and the BorderScrollPadding start offset.

Bug: 1045599
Change-Id: Idb4d550f0d8b2aca306945a46d41b5a30a20bca4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622586
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Ethan Jimenez <ethavar@microsoft.com>
Reviewed-by: Kurt Catti-Schmidt <kschmi@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#845494}

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wpt-commits: c71b45a1c65da368339f4db424a3740d7f8ea7f3
wpt-pr: 27158