Update iframe's height even if it's zero.

Since we're able to have zero-height iframes consider you have an invisible one (height == 0). And parent page sent some event to it after which some element in the iframe is appeared. Now, if you'll hide that element back...
**before this commit:** `seamless_update` event ignore real (0) height and iframe still overlap page elements but becomes invisible (I've waste a much time with that).
**with this commit:** `seamless_update` event set height to real iframe's inner height which is 0.