Add ve.now utility function

On browsers that implement the Navigation Timing API, performance.now()
provides values with microsecond precision that are guaranteed to be monotonic
(i.e., they are not subject to skew due to changes to the system clock).
This patch adds a `ve.now` utility function that will use this API when it is
available and fall back gracefully to `Date.now` when it is not.

Change-Id: I377025fcb23cb26399b9e437e33c8afa138916af
Merge "Add ve.now utility function"