Remove libs/clock in favor of luci-go/common/clock.

go/src/infra/libs/clock/systemtimer.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"
-)
-
-// A Timer implementation that uses time.Timer.
-type systemTimer struct {
-        T *time.Timer // The underlying timer. Starts as nil, is initialized on Reset.
-}
-
-var _ Timer = (*systemTimer)(nil)
-
-func (t *systemTimer) GetC() (c <-chan time.Time) {
-        if t.T != nil {
-                c = t.T.C
-        }
-        return
-}
-
-func (t *systemTimer) Reset(d time.Duration) bool {
-        if t.T == nil {
-                t.T = time.NewTimer(d)
-                return false
-        }
-        return t.T.Reset(d)
-}
-
-func (t *systemTimer) Stop() bool {
-        if t.T == nil {
-                return false
-        }
-        return t.T.Stop()
-}