Don't allow replaceWithChildrenElements for integration points

[evilpie]

Basically the same idea as #382. Parsing behavior changes at HTML integration point and MathML text integration point.

[mozfreddyb]

Some findings:
In contrast to integration points, there are way to "escape" integration in the The rules for parsing tokens in foreign content

[mozfreddyb]

Example test case<svg><font color="red"><clippath>, whith the attribute "color" being removed by the sanitizer.

During parsing, the "color" being present makes the following elements escape SVG. If you sanitizer during parsing, what happens?

If you sanitize on insertion, we will parse as we already do (not sure)?