Address review actions:
* In the gecko vbufBackend: comment why two sets of table attributes are being added.
* browseModeDocumentTreeInterceptor._getNearestTableCell: make the retry limit configurable via a _missingTableCellSearchLimit property.
* browseModeDocumentTreeInterceptor._getNearestTableCell: try more cells for any LookupError not just hiddenTableCellFound (browsers such as chrome do not expose hidden cells).
* remove browseMode.HiddenTableCellFound
* Gecko_ia2 virtualBuffer's _getTableCellAt: raise LookupError if a hidden cell is found rather than HiddenTableCellFound
