Move sans-serif fallback font higher than emoji fonts (#15855)

Move sans-serif fallback font higher than emoji fonts (#15855)

The Tor browser does not use the system-ui font and no other fonts in the stack match
its default fonts. In fact it is possible that it will in future only
match generic fonts. This means that all rendering will first try the
emoji fonts before falling back to the sans-serif font for glyphs.

In this case has the emoji fall back fonts for Tor contains empty glyphs
for numbers - in order to protect privacy - and leads to numbers being
rendered as empty glyphs. This is clearly not ideal and whilst we could
use the Arimo font - as I state above I suspect that Tor will eventually
ban detecting this and we should instead move the sans-serif font higher
in the stack so that it matches before the emoji fonts.

Partial fix of #15844

Signed-off-by: Andrew Thornton <art27@cantab.net>