internal/trace/internal/testgen: fix missing stacks nframes arg

Change-Id: I6a6a6964c9c1322bfe289394d5d3937d1f7097bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/694616
Reviewed-by: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: David Chase <[email protected]>
Auto-Submit: Michael Knyszek <[email protected]>

Author: felixge

src/internal/trace/internal/testgen/trace.go

@@ -295,7 +295,7 @@ func (g *Generation) writeEventsTo(tw *raw.TextWriter) {
 	b.RawEvent(tracev2.EvStacks, nil)
 	for stk, id := range g.stacks {
 		stk := stk.stk[:stk.len]
-		args := []uint64{id}
+		args := []uint64{id, uint64(len(stk))}
 		for _, f := range stk {
 			args = append(args, f.PC, g.String(f.Func), g.String(f.File), f.Line)
 		}