Add the SharedWorker.workerStart property for high resolution timers.

Update the Worker implementation of the high resolution timer spec
(v2), supporting the workerStart attribute which provides the
difference between the time origins of a shared worker and the
document it is accessed from

 https://dvcs.w3.org/hg/webperf/raw-file/tip/specs/HighResolutionTime2/Overview.html#sec-worker-start

R=tonyg,abarth
BUG=169318

Review URL: https://codereview.chromium.org/111743007

git-svn-id: svn://svn.chromium.org/blink/trunk@164535 bbb929c8-8fbe-4397-9dbb-9b2b20218538