Add a unit test for timers

      This test will help to ensure current behavior isn't altered
while removing the timer heap.

Original patch by Alex Clarke <alexclarke@chromium.org>.

BUG=463143

Review URL: https://codereview.chromium.org/1130423002

git-svn-id: svn://svn.chromium.org/blink/trunk@195235 bbb929c8-8fbe-4397-9dbb-9b2b20218538