internal/trace: actually add experimental data to map

internal/trace: actually add experimental data to map

addExperimentalData was missing actually placing the data into the map,
leaving ExperimentalEvent.Data always nil.

Change-Id: I1715ab11eb7cfcdb982b56be28378521a357ca2d
Reviewed-on: https://go-review.googlesource.com/c/go/+/588955
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>