Fix QAccessible name and description calculation

The logic was removed in
1881f56d

.

There is no platform independent implementation in Chromium for
calculating Accessible name which follow the W3C Accessibility standard.
We need to apply this logic in order to get Accessible name
and description correctly.

It also fixes the QWebEngineAccessibility test cases.

Change-Id: I733c6311301c43610b7a9aaf22c73c2e5d368c70
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>