image/jpeg: clean up BenchmarkDecode and BenchmarkEncode to not refer to opacity. Those references were copy/pasted from the image/png encoding benchmarks, which cares whether or not the source image is opaque, but the JPEG encoder does not care.
image/jpeg: clean up BenchmarkDecode and BenchmarkEncode to not
refer to opacity. Those references were copy/pasted from the
image/png encoding benchmarks, which cares whether or not the
source image is opaque, but the JPEG encoder does not care.

R=r
CC=golang-dev
http://codereview.appspot.com/6623052

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang