Mac is the only holdout that performs non-composited fades of page overlays. We should adopt that mechanism (as a precursor to doing a CA-accelerated fade) and remove the now-unnecessary property.
Created attachment 207960 [details]
Created attachment 207985 [details]
We also need to make WKBundlePageOverlayFractionFadedIn return 1 always, to prevent clients from attempting to bake the fade opacity in.
And, make the find-in-page overlay not bake the opacity in as well.
In doing this I also removed PageOverlay::fractionFadedIn so nobody tries to use it for anything (since it is not useful with composited fade, and will be totally wrong with CA-accelerated fade).