Add the experiment flag of EXT_REFS
Add the experiment flag of EXT_REFS

This experiment will add 3 more reference frames and support
bi-predictive frames.

Change-Id: I76ce9b25f2fa4c47c9cef906ee6aab5869337f2f

luctrudeau/aom,GrokImageCompression/aom,mbebenita/aom,luctrudeau/aom,luctrudeau/aom,luctrudeau/aom,GrokImageCompression/aom,GrokImageCompression/aom,smarter/aom,mbebenita/aom,mbebenita/aom,luctrudeau/aom,mbebenita/aom,smarter/aom,luctrudeau/aom,mbebenita/aom,smarter/aom,mbebenita/aom,GrokImageCompression/aom,mbebenita/aom,smarter/aom,GrokImageCompression/aom,mbebenita/aom,smarter/aom,smarter/aom,mbebenita/aom,GrokImageCompression/aom