Is it possible to include line breaks in the description of a page shown in Google search results?
No. You can't use html; and HTML ignores whitespace (when it's not used inside a text tag) so it doesn't matter if there's a space (\n\r,etc) or not; google parser will just grab the string, strip any bad code and place the text alone.
Take into consideration that Google will grab 150 characters from the description meta tag. You can test it here: http://www.javascriptkit.com/script/script2/charcount.shtml