Move HighResolutionTimeInWorkers to stable.

This moves the "High Resolution Time Level 2" mini feature to stable,
extending high resolution timing support to also cover Web Workers,

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

Relevant "Intent to Ship" thread,

 https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/R3OiFdzqm1w

R=jochen@chromium.org
BUG=169318

Review URL: https://codereview.chromium.org/153543004

git-svn-id: svn://svn.chromium.org/blink/trunk@166435 bbb929c8-8fbe-4397-9dbb-9b2b20218538