[HighResolutionTime] New worker tests added

Hi,

I've added 2 basic tests for HighResolutionTime level 2. They check that
both types of workers support performance.now() and that the values are
reasonable relative to the main document.

http://www.w3c-test.org/webperf/tests/submission/Google/HighResolutionTime/

They should pass with Chrome from Dev Channel started
with --enable-experimental-web-platform-features. Note that we no longer
use vendor prefixes and keep new features hidden behind that flag until the
spec and implementation are stable.

James