cmd/coordinator: tell browsers not to sniff content of log messages

Chrome and other browsers like to suppress output while they sniff the
content; tell them not to so we see the logs immediately.

Fixes golang/go#10097

Change-Id: I73295a46ff803c1e3f8a092afb225caaa28198bb
Reviewed-on: https://go-review.googlesource.com/7440
Reviewed-by: Andrew Gerrand <adg@golang.org>
diff --git a/cmd/coordinator/coordinator.go b/cmd/coordinator/coordinator.go
index 1492ed9..c77e005 100644
--- a/cmd/coordinator/coordinator.go
+++ b/cmd/coordinator/coordinator.go
@@ -500,6 +500,7 @@
 		return
 	}
 	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
+	w.Header().Set("X-Content-Type-Options", "nosniff")
 	writeStatusHeader(w, st)
 
 	w.(http.Flusher).Flush()