There’s an interesting post on ars technica on "Unwrapping Lollipop" talking to "high ranking members of the Android team" about changes to the OS. It includes a very useful breakdown of what’s now in the Android OS, what’s in Play services and what’s distributed via the Play Store.
Of particular interest is that WebView has been unbundled and now comes from the Play Store. The idea is to be able to more-easily (auto) update WebView for performance and security reasons. However, this won’t be for pre-Lollipop devices. In the near-term, there will still be a large number of old devices on old and varied versions of WebView. Also, unbundling has the side-effect that, going forward, non-Google sanctioned, AOSP-derived devices won’t have the latest WebView. Google will obviously care less about this but it will affect the 20% of developers on those platforms.