Add shouldBeNow() to js-tests.js in LayoutTests.

The function checks that an expression returns the current time. The
check is non-trivial, given that time might go backwards due to NTP
updates.

BUG=320686

Review URL: https://codereview.chromium.org/69813015

git-svn-id: svn://svn.chromium.org/blink/trunk@162645 bbb929c8-8fbe-4397-9dbb-9b2b20218538