Found this during code review:
https://chromium-review.googlesource.com/c/chromium/src/+/997096/2/chrome/browser/chromeos/extensions/wallpaper_private_api.cc#b501

wallpaperPrivate.resetWallpaper never calls SendResponse. Note that even if the function does not have a callback, we ought to call SendResponse for *all* ExtensionFunctions.
I'm going to fix this while migrating the function from AsyncExtensionFunction to UIThreadExtensionFunction.