runtime: delete UpdateMemStats, replace with ReadMemStats(&stats).

Unexports runtime.MemStats and rename MemStatsType to MemStats.
The new accessor requires passing a pointer to a user-allocated
MemStats structure.

Fixes #2572.

R=bradfitz, rsc, bradfitz, gustavo
CC=golang-dev, remy
https://golang.org/cl/5616072
diff --git a/src/pkg/expvar/expvar.go b/src/pkg/expvar/expvar.go
index ee32eff..0ccfb34 100644
--- a/src/pkg/expvar/expvar.go
+++ b/src/pkg/expvar/expvar.go
@@ -277,7 +277,9 @@
 }
 
 func memstats() interface{} {
-	return runtime.MemStats
+	stats := new(runtime.MemStats)
+	runtime.ReadMemStats(stats)
+	return *stats
 }
 
 func init() {