cssEscaper escapes using the CSS convention: `\` + hex + optional-space

It outputs the space when the escape could be followed by
a hex digit to distinguish a "\na" from "\u00aa".

It did not output a space when the escape is followed by a space
character so did not distinguish "\n " from "\n".

Currently when doing lookahead, it does not distinguish spaces that
will be escaped later by the same function from ones that will not.
This is correct but suboptimal.

R=nigeltao
CC=golang-dev
https://golang.org/cl/5306042