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This patch implements alignment within a grid-item's grid-area.
When creating the GridItemData we read of the containers, and
grid-item's style, or determine its alignment axis-edge, and if it
should be stretched, saving this information on the GridItemData.

When placing the grid-item it will use this to adjust the offset.

This doesn't implement baseline alignment which requires a secondary
pass to correctly determine the baseline for the column/row.

The other TODO relates to the constraint-space API, and how grid will
need a slightly new concept in order to do block-axis stretching
easily. By default non-replaced grid-items will stretch to the available
space. A new bit on the space can indicate that we should stretch to
the available space (if possible), this is similar to the shrink-to-fit
bit we have today.

This adds a self-start/self-end test as after implementing this now new
tests passed immediately, and I don't believe we had good test
coverage for inflow content with these keywords.

Bug: 1045599
Change-Id: I8c9cc51381c3d75519e52b32472b58ec96a6a85a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2492176
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Kurt Catti-Schmidt <kschmi@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#821043}