Conditionally log key operations against DOMTimers.

This is needed to diagnose intermittent failures on the tryservers.

BUG=408358

Review URL: https://codereview.chromium.org/516103002

git-svn-id: svn://svn.chromium.org/blink/trunk@181067 bbb929c8-8fbe-4397-9dbb-9b2b20218538