time: record monotonic clock reading in time.Now, for more accurate comparisons

See https://golang.org/design/12914-monotonic for details.

Fixes #12914.

Change-Id: I80edc2e6c012b4ace7161c84cf067d444381a009
Reviewed-on: https://go-review.googlesource.com/36255
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Caleb Spare <cespare@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/src/time/zoneinfo_read.go b/src/time/zoneinfo_read.go
index 19cd40d..1b3356e 100644
--- a/src/time/zoneinfo_read.go
+++ b/src/time/zoneinfo_read.go
@@ -188,7 +188,7 @@
 
 	// Fill in the cache with information about right now,
 	// since that will be the most common lookup.
-	sec, _ := now()
+	sec, _, _ := now()
 	for i := range tx {
 		if tx[i].when <= sec && (i+1 == len(tx) || sec < tx[i+1].when) {
 			l.cacheStart = tx[i].when