A Multicenter, Randomized, Open-label, Phase II Study to Evaluate the Efficacy and Safety of Erlotinib Versus Etoposide Plus Cisplatin With Concurrent Radiotherapy in Unresectable Stage III Non-small Cell Lung Cancer (NSCLC) With Activating Mutation of Epidermal Growth Factor Receptor (EGFR) in Exon 19 or 21