Add context-aware "time" library wrapper.

go/src/infra/gae/libs/wrapper/mathrand.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 wrapper
 
 import (
         "math/rand"
 
         "golang.org/x/net/context"
+        "infra/libs/clock"
 )
 
 // MathRandFactory is the function signature for factory methods compatible with
 // SetMathRandFactory.
 type MathRandFactory func(context.Context) *rand.Rand
 
 // GetMathRand gets a *"math/rand".Rand from the context. If one hasn't been
 // set, this creates a new Rand object with a Source initialized from the
-// current time accordint to GetTimeNow(c).UnixNano().
+// current time clock.Now(c).UnixNano().
 func GetMathRand(c context.Context) *rand.Rand {
         if f, ok := c.Value(mathRandKey).(MathRandFactory); ok && f != nil {
                 return f(c)
         }
-        return rand.New(rand.NewSource(GetTimeNow(c).UnixNano()))
+        return rand.New(rand.NewSource(clock.Now(c).UnixNano()))
 }
 
 // SetMathRandFactory sets the function to produce *"math/rand".Rand instances,
 // as returned by the GetMathRand method.
 func SetMathRandFactory(c context.Context, mrf MathRandFactory) context.Context {
         return context.WithValue(c, mathRandKey, mrf)
 }
 
 // SetMathRand sets the current *"math/rand".Rand object in the context. Useful
 // for testing with a quick mock. This is just a shorthand SetMathRandFactory
 // invocation to set a factory which always returns the same object.
 func SetMathRand(c context.Context, r *rand.Rand) context.Context {
         if r == nil {
                 return SetMathRandFactory(c, nil)
         }
         return SetMathRandFactory(c, func(context.Context) *rand.Rand { return r })
 }