Add Context cancellation to clock.

common/clock/tags.go

+// Copyright 2016 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 (
+        "golang.org/x/net/context"
+)
+
+var clockTagKey = "clock-tag"
+
+// Tag returns a derivative Context with the supplied Tag appended to it.
+//
+// Tag chains can be used by timers to identify themselves.
+func Tag(c context.Context, v string) context.Context {
+        return context.WithValue(c, &clockTagKey, append(Tags(c), v))
+}
+
+// Tags returns a copy of the set of tags in the current Context.
+func Tags(c context.Context) []string {
+        if tags, ok := c.Value(&clockTagKey).([]string); ok && len(tags) > 0 {
+                tclone := make([]string, len(tags))
+                copy(tclone, tags)
+                return tclone
+        }
+        return nil
+}