Use const cdf with aom_read_symbol().
Use const cdf with aom_read_symbol().

Change-Id: I6e60d9083da8a2d8f7e182e4f12704eddd170df6

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