exp/html: adjust parseForeignContent to match spec

exp/html: adjust parseForeignContent to match spec

Remove redundant checks for integration points.

Ignore null bytes in text.

Don't break out of foreign content for a <font> tag unless it
has a color, face, or size attribute.

Check for MathML text integration points when breaking out of
foreign content.

Pass two new tests.

R=nigeltao
CC=golang-dev
https://golang.org/cl/6256045