It seems that the m_ascent member is not really needed as the ascent can be easily calculated in firstLineBaseLine() after some adjustment with how the attribute values are resolved. I was not able to find a testcase that shows how it improves dynamic update. Actually, it seems that we no longer need to override styleDidChange and updateFromElement so I'll remove them in that bug too.
Created attachment 286850 [details]
Comment on attachment 286850 [details]
Clearing flags on attachment: 286850
Committed r205098: <http://trac.webkit.org/changeset/205098>
All reviewed patches have been landed. Closing bug.