Prot# GS-US-236-0115: A Phase 3b Randomized, Open Label Study to Evaluate Switching from Regimens Consisting of a Ritonavir-boosted Protease Inhibitor (PI/r) plus Emtricitabine/Tenofovir Fixed-Dose Combination (FTC/TDF) to the Elvitegravir/Cobicistat/ Emt