Add Context cancellation to clock.

common/clock/tags.go

Index: common/clock/tags.go
diff --git a/common/clock/tags.go b/common/clock/tags.go
new file mode 100644
index 0000000000000000000000000000000000000000..8f15aeb954b9645948eeaf2cb6fb70a37c279293
--- /dev/null
+++ b/common/clock/tags.go
@@ -0,0 +1,28 @@
+// 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
+}