Add ability to run async magnitude-perf tests.

I would like to add new time complexity test.
Unfortunately, current framework doesn't support async tests.

In this patch I wrap for and while operators, so they could
run asynchronously.

CL I'm going to write test to (fix n^2 HTML parsing):
https://codereview.chromium.org/494993002/

Here is the first async test:
https://codereview.chromium.org/500363002/

BUG=410790

Review URL: https://codereview.chromium.org/544453004

git-svn-id: svn://svn.chromium.org/blink/trunk@181539 bbb929c8-8fbe-4397-9dbb-9b2b20218538