[nanobench] Report the ratio of median/min.

Bug: skia:12046
Change-Id: If47bb7c5ddfa19412dc058c9da8d2a1153167851
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/414337
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Joe Gregorio <jcgregorio@google.com>

bench/nanobench.cpp

@@ -1416,6 +1416,7 @@ int main(int argc, char** argv) {
 
             // Metrics
             log.appendMetric("min_ms", stats.min);
+            log.appendMetric("min_ratio", sk_ieee_double_divide(stats.median, stats.min));
             log.beginArray("samples");
             for (double sample : samples) {
                 log.appendDoubleDigits(sample, 16);