Add context-aware "time" library wrapper.

go/src/infra/libs/clock/testclock/context.go

Index: go/src/infra/libs/clock/testclock/context.go
diff --git a/go/src/infra/libs/clock/testclock/context.go b/go/src/infra/libs/clock/testclock/context.go
new file mode 100644
index 0000000000000000000000000000000000000000..9ea91fc2e30459cb27942831e7cc38f5732929ef
--- /dev/null
+++ b/go/src/infra/libs/clock/testclock/context.go
@@ -0,0 +1,20 @@
+// 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
+}