html: stop at scope marker node when generating implied </a> tags
A <a> tag generates implied end tags for any open <a> elements.
But it shouldn't do that when it is inside a table cell the the open <a>
is outside the table.
So stop the search for an open <a> when we reach a scope marker node.
Pass tests1.dat, test 78:
<a href="blah">aba<table><tr><td><a href="foo">br</td></tr>x</table>aoe
| <html>
| <head>
| <body>
| <a>
| href="blah"
| "abax"
| <table>
| <tbody>
| <tr>
| <td>
| <a>
| href="foo"
| "br"
| "aoe"
Also pass test 79:
<table><a href="blah">aba<tr><td><a href="foo">br</td></tr>x</table>aoe
R=nigeltao
CC=golang-dev
https://golang.org/cl/
5320063