Indra Nooyi became C.E.O. of PepsiCo just in time for a global financial meltdown. She also had a portfolio full of junk food just as the world decided that junk food is borderline toxic. Here's the story of how she overhauled that portfolio, stared down activist investors, and learned to "leave the crown in the garage." (Part 3 of a special series, "The Secret Life of C.E.O.'s")