Just tested (using ejj.io/SOP.php), Javelin on my Note 3 isn't vulnerable.
So this will automatically be Chrom(ium) on newer versions of Android?
Yeap, it is already Chromium WebView since 4.4 :) But the performance lags behind bleeding edge Chrome since Google gives its own Chrome a more advanced version of Chromium WebView.
How did you handle file chooser? As far as I know, WebView disabled fileChooser by 4.4 [1]. This is a problem which we came across today.