Add context-aware "time" library wrapper.

go/src/infra/libs/clock/systemclock.go

Index: go/src/infra/libs/clock/systemclock.go
diff --git a/go/src/infra/libs/clock/systemclock.go b/go/src/infra/libs/clock/systemclock.go
new file mode 100644
index 0000000000000000000000000000000000000000..b5384a411fed547f99b9067c52abd09f2d2b38b6
--- /dev/null
+++ b/go/src/infra/libs/clock/systemclock.go
@@ -0,0 +1,39 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package clock
+
+import (
+	"time"
+)
+
+// Implementation of Clock that uses Go's standard library.
+type systemClock struct{}
+
+// System clock instance.
+var systemClockInstance systemClock
+
+var _ Clock = systemClock{}
+
+// GetSystemClock returns an instance of a Clock whose method calls directly use
+// Go's "time" library.
+func GetSystemClock() Clock {
+	return systemClockInstance
+}
+
+func (systemClock) Now() time.Time {
+	return time.Now()
+}
+
+func (systemClock) Sleep(d time.Duration) {
+	time.Sleep(d)
+}
+
+func (systemClock) NewTimer() Timer {
+	return new(systemTimer)
+}
+
+func (systemClock) After(d time.Duration) <-chan time.Time {
+	return time.After(d)
+}