cmd/coordinator: fix pinging of dashboard to show in-progress builds

The blue in-progress gophers have been missing in build.golang.org
because the magic log span strings changed.

Change-Id: I702883d8a6f6eea31aaaaa25ebe10e929992a677
Reviewed-on: https://go-review.googlesource.com/22747
Reviewed-by: Ross Light <light@google.com>

cmd/coordinator/coordinator.go

@@ -2673,7 +2673,7 @@ func (st *buildStatus) logEventTime(event string, optText ...string) {
 	st.mu.Lock()
 	defer st.mu.Unlock()
 	switch event {
-	case "creating_gce_instance", "got_machine", "got_buildlet":
+	case "finish_get_buildlet", "create_gce_buildlet":
 		if !st.startedPinging {
 			st.startedPinging = true
 			go st.pingDashboard()