Web Replay: add option that forces scrolling to be deterministic.

Threaded scrolling in the mac port uses a thread-safe scroll tree
data structure to asynchronously update and render scrolls. The
new code paths for threaded scrolling avoid WebPage and
UserInputProxy, which is how we capture and simulate wheel events.

While scrolling is deoptimized to the main thread when wheel
event handlers exist, replay needs to capture *all* scrolls
whether they could fire event handlers or not. So, we added a
flag that effectively forces scrolling to always deoptimize, and
set it active during capturing only. Once the scrolls have been
captured, they can be replayed on the main thread even if new
scrolls would have been directed to the scrolling thread.