view-transitions: Use default transform-origin for group elements

The spec text for computing the transform applied to
::view-transition-group elements is as follows:

"A transform that would map element’s border box from the snapshot root
origin to its current visual position."

This value needs to be relative to the center of the element's border
box. The current implementation computes the transform relative to the
element's top left corner with an additional UA style rule for
transform-origin to account for this. This is inconsistent with the UA
styles defined in the spec.

This change fixes the above by computing the transform relative to the
element's border bounding box center and removes the incorrect UA style
rule.

R=bokan@chromium.org, vmpstr@chromium.org

Fixed: 1420369
Change-Id: I0ee77266f29f880e8651111129e832b37eea5c48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4299931


Commit-Queue: Khushal Sagar <khushalsagar@chromium.org>
Reviewed-by: Vladimir Levin <vmpstr@chromium.org>
Auto-Submit: Khushal Sagar <khushalsagar@chromium.org>
Commit-Queue: Vladimir Levin <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1112860}