Add context-aware "time" library wrapper.

go/src/infra/libs/clock/clock.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"
+)
+
+// A Clock is an interface to system time.
+//
+// The standard clock is SystemClock, which falls through to the system time library.
+// Another clock, FakeClock, is available to simulate time facilities for testing.
+type Clock interface {
+        Now() time.Time                       // Returns the current time (see time.Now).
+        Sleep(time.Duration)                  // Sleeps the current goroutine (see time.Sleep)
+        NewTimer() Timer                      // Creates a new Timer instance.
+        After(time.Duration) <-chan time.Time // Waits a duration, then sends the current time.
+}