Add ratio calculation to take the ratio of two calculations.

perf/go/parser/parser_test.go

Index: perf/go/parser/parser_test.go
diff --git a/perf/go/parser/parser_test.go b/perf/go/parser/parser_test.go
index 71fbebf5790e8fea8292a02584a59d684212c3ad..6e0c4a784471df598e5538ebb48c5399d1356224 100644
--- a/perf/go/parser/parser_test.go
+++ b/perf/go/parser/parser_test.go
@@ -131,6 +131,10 @@ func TestAve(t *testing.T) {
 	}
 }
 
+func TestRatio(t *testing.T) {
+	ctx := newTestContext()
+}
+
 func TestFill(t *testing.T) {
 	ctx := newTestContext()
 	ctx.Tile.Traces["t1"].(*types.PerfTrace).Values = []float64{1e100, 1e100, 2, 3, 1e100, 5}