Implement the foster-parenting algorithm for content that is inside a table
but not in a cell.

Also fix a bug in reconstructing the active formatting elements.

Pass test 30 in tests1.dat:
<a><table><td><a><table></table><a></tr><a></table><b>X</b>C<a>Y

R=nigeltao
CC=golang-dev
https://golang.org/cl/5309052