exp/html: fix foster-parenting when elements are implicitly closed

exp/html: fix foster-parenting when elements are implicitly closed

When an element (like <nobr> or <p>) was implicitly closed by another
start tag, it would keep foster parenting from working because
the check for what was on top of the stack of open elements was
in the wrong place.

Move the check to addChild.

Pass 2 additional tests.

R=nigeltao
CC=golang-dev
https://golang.org/cl/6460045