Fix many-draw-calls.html timeouts on some slow platforms

Timeouts could be seen frequently on a debug build of a browser on a
slower ARM platform when running the previous version of the test. The
new version tells the harness it is alive every ten iterations,
preventing the test from timing out.
Merge pull request #443 from NVIDIA/many-draw-calls-timeout-fix

Fix many-draw-calls.html timeouts on some slow platforms