Add context-aware "time" library wrapper.

go/src/infra/libs/clock/testclock/context.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 testclock
+
+import (
+        "time"
+
+        "golang.org/x/net/context"
+        "infra/libs/clock"
+)
+
+// UseTime instantiates a TestClock and returns a Context that is configured to
+// use that clock, as well as the instantiated clock.
+func UseTime(ctx context.Context, now time.Time) (rctx context.Context, tc TestClock) {
+        tc = New(now)
+        rctx = clock.Set(ctx, tc)
+        return
+}