NOBACKPAINT actually means something else: Do not use back buffer to avoid flickering.
It is completely completely ignored with DHCtrl.
Now for your issue: This will be more related to improper implementation of Chromium. Recently, I needed browser in one of my apps, I have started with Bazaar IE package, but found somewhat similar issues (flickering). I was able to fix them by fixing the package. I guess something like that is needed for Chromium Browser package....