Add context-aware "time" library wrapper.

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

+// 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)
+}