runtime: fix TestGcLastTime on windows

Intended to fix:
http://build.golang.org/log/d6718ea67541b8c6be2bb14bcbc4e1c4261f67d7

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews
https://golang.org/cl/127520043

src/pkg/runtime/gc_test.go

@@ -165,7 +165,8 @@ func TestGcLastTime(t *testing.T) {
 		t.Fatalf("bad last GC time: got %v, want [%v, %v]", last, t0, t1)
 	}
 	pause := ms.PauseNs[(ms.NumGC+255)%256]
-	if pause == 0 || pause > 10e9 {
+	// Due to timer granularity pause can actually be 0 on windows.
+	if (pause == 0 && runtime.GOOS != "windows") || pause > 10e9 {
 		t.Fatalf("bad last GC pause: got %v, want [0, 10e9]", pause)
 	}
 }