Fix non-subpixel layout build.

There is no postfix++ operator for the LayoutUnit type. But there is a
prefix one, so use that.
Don't try to be smart with guessing MIME types

Trying to guess the MIME type from an URL is not very safe. When
handling a 302 redirection, the HTTP server doesn't serve us a mime
type, and we'll get one later on at the new location. However, if we
already set the MIME type for the request at this point, the actual one
will be ignored.

Fixes #9396, and a few other places where this led to downloading files
instead of displaying them as html.