When Rihanna tapped her abusive ex-boyfriend, Chris Brown, to join her on a sexually explicit remix of "Birthday Cake" last month, the music world was shocked. Chatting with Ryan Seacrest on his radio show, she spoke about the reasons behind that remix and their other collaboration (on Brown's "Turn Up the Music"). "I reached out to him about doing 'Birthday Cake' because [he was] the only person that really made sense to do the record, just ...