Add Timing interface

Add Timing interface

* Add `Timing`, an interface which mimics the W3C User Timing API.
  It provides a canonical way to store and retrieve markers (timestamps)
  and measures (timestamps + duration).
* As the initial use-case, use it to record 'requestShutdown'.

Change-Id: I36b29162ffcc091406df025463b0e2797e52f19a