From Hangout's chat (abbeviated):
Mike Rolig: @Alex Russell  is there a way for a localhost demo on Android to test the splash-screen code path? If so, how do I debug why a Web APK isn't triggering fullscreen mode? I'm able to use "add to homescreen" to install a web apk, but it won't open full screen.

Yaron Friedman: I suspect it's because the intent doesn't match intent filter
worth trying *.corp.google.com to see if it helps
otherwise if you're using server-build webapks, it should just work
I'm also not aware of a reason why localhost wouldn't but that's my guess

Marking as WontFix and filed internal bug