It looks like this is happening because WPE doesn't have a DragImageWPE.cpp file.
Created attachment 325533 [details] Patch
Comment on attachment 325533 [details] Patch Well I got WPE working, but looks like it broke macOS and iOS. I don't plan to debug that. :)
Oh, the fix for iOS/Mac is to delete the lines that say "DragImage.cpp in Sources" in WebCore.xcodeproj/proj.pbxproj
Created attachment 325587 [details] Patch
Looks like it works. mac-debug might be about to complain about a flaky test, though.
Comment on attachment 325587 [details] Patch r=me.
Comment on attachment 325587 [details] Patch Clearing flags on attachment: 325587 Committed r224339: <https://trac.webkit.org/changeset/224339>
All reviewed patches have been landed. Closing bug.
<rdar://problem/35621445>