Currently, we support two implementations of DropdownPopupWindow:

DropdownPopupWindowImpl https://cs.chromium.org/chromium/src/ui/android/java/src/org/chromium/ui/DropdownPopupWindowImpl.java

DropdownPopupWindowJellyBean
https://cs.chromium.org/chromium/src/ui/android/java/src/org/chromium/ui/DropdownPopupWindowJellyBean.java

As the name suggests, the latter should only be necessary on Jellybean, but the blocking bug crbug.com/838736 currently means we are also using this on KitKat and Marshmallow.

After fixing that bug and reducing the use of the deprecated code to just Jellybean, the only thing blocking the removal of the second implementation will be the end of support for JB.