Hearthpwn

This deck gives you a few key tools for dealing with Chromaggus' otherwise crippling hero power.  The most crucial of these is Summoning Stone, which will turn his constant feeding you spells into your own strength, as you turn each into a 3-cost minion of your own.  

Until then, your job is to establish strong early game tempo and get board control as of Turn 5. To do that, mulligan for Doomsayer, Shadow Word: Pain and Silence.  All of these will help you make sure there's nothing on board when you start building your own Summoning Stone army.  

If the game does go late, you'll have free Arcane Giants to put on board, thanks to all the spells you've been spitting out every turn up until then.  Chromaggus doesn't have any hard removal, so these tend to stick around.  Especially if buffed using your ample buffs.

And if for some reason the game manages to go beyond there, you've got Yogg to back you up at the top end.  

But honestly, as long as you establish early game tempo, then summon 3-drops for a few turns, you should be able to just trade yourself to victory with all the minions you're constantly spitting out.