Add Context cancellation to clock.

common/clock/timer.go

 // Copyright 2015 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"
 )
 
 // Timer is a wrapper around the time.Timer structure.
 //
 // A Timer is instantiated from a Clock instance and started via its Reset()
 // method.
 type Timer interface {
-        // GetC returns the underlying timer's channel, or nil if the timer is no
-        // running.
-        GetC() <-chan time.Time
+        // GetC returns the underlying timer's channel.
+        GetC() <-chan AfterResult
 
         // Reset configures the timer to expire after a specified duration.
         //
         // If the timer is already running, its previous state will be cleared and
         // this method will return true.
         Reset(d time.Duration) bool
 
         // Stop clears any timer tasking, rendering it inactive.
         //
         // Stop may be called on an inactive timer, in which case nothing will happen
         // If the timer is active, it will be stopped and this method will return true.
         Stop() bool
 }