nanobench does not need to handle failed rewind.

Now that all SkCodecs can rewind (assuming the stream is rewindable),
we do not need to special case it.

Pointed out by Derek in the code review that added this.

TBR=djsollen

Review URL: https://codereview.chromium.org/1058633002

bench/nanobench.cpp

@@ -661,10 +661,6 @@ public:
                     case SkImageGenerator::kInvalidConversion:
                         // This is okay. Not all conversions are valid.
                         break;
-                    case SkImageGenerator::kCouldNotRewind:
-                        // FIXME: This is due to a bug in some implementations
-                        // of SkCodec. All should support rewinding.
-                        break;
                     default:
                         // This represents some sort of failure.
                         SkASSERT(false);