I'd like to see a custom version of Chromium for this purpose. Google's sandboxing is great. Just reduce the attack surface by stripping out non-essentials like plugins, SVG, WebGL, NaCl, etc. etc. and you have a pretty darn secure browser. See the ridiculous complexity of those two exploits by Pinkie Pie for what attackers are up against.

Seems like a good convenience/security tradeoff to me.