[views-ax] Implement ViewAccessibilityAXTreeSource::SerializeNode

This CL implements the SerializeNode function, which mostly completes
the implementation of the ViewAccessibilityAXTreeSource (% the focus
handling logic).

Serializing is straightforward now that the ViewsAX cache migration
is completed -- we simply have to call
ViewAccessibility::GetAccessibleNodeData on the view or virtual view.

Bug: 40672441
Change-Id: I6fbe6dc09fb6a7849dd03de9bfa6c526c68b96c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6684846
Commit-Queue: Javier Contreras <javiercon@microsoft.com>
Auto-Submit: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Reviewed-by: Javier Contreras <javiercon@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1480607}