[a11y] Validate kDescription attr after merging AXNodeData members

A check for `kDescription` has been moved after the call to
`ViewAccessibility::Merge`, so that we only check the `data`
after all the attributes have been set.
Otherwise, there was a bug where the description was not yet
populated into the out `data` member in `Merge` and so we were
falling into the `if` block below, which led to hangs when opening up
the bookmarks bar in Chrome.

Bug: 326509144
Change-Id: I14f4c9787ca587387a6e2a970e429784488d356d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5397530
Reviewed-by: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Commit-Queue: Javier Contreras <javiercon@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1278698}