No more false stall alarms from the macOS app window

Obeya logs every moment a page freezes for half a second or more. In the macOS app window it logged a freeze almost every second, all night long: WebKit slows a page's timers when the window is visible but not in front, and the page took each slow tick for a freeze. Pages now check against a heartbeat from a background thread that WebKit doesn't slow down, so only real freezes are logged. A page that freezes the same way again and again gets a single entry plus a count, so the log keeps the server's own freezes.