[fuchsia][a11y] Fill transform unconditionally in node updates.

This change is necessary to ensure that we correctly handle the case
where a node's transform is updated to be the identity matrix.

Bug: fuchsia:83958
Test: AXTreeConverterTest
Change-Id: I3569e8369aea6b654e02585766922eb384431f2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3140430


Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Alexander Brusher <abrusher@google.com>
Cr-Commit-Position: refs/heads/main@{#917745}