displayDidRefresh is currently sent over EventDispatcher, or WebProcess, depending on which consumers are registered. Bug 232918 is going to add another consumer that wants the background-thread EventDispatcher behaviour, so it's simpler to just always do that.
Pull request: https://github.com/WebKit/WebKit/pull/13777
<rdar://problem/109535049>
This can be a performance regression when we're only using it on the main thread, closing.