bugfix: set iframe's dimension to avoid some ui problem in ies

In ie7, if runner's iframe is 0*0, js cannot know if a dom element is visible, any element always be considered as unvisible, and in ie6789,  this would cause a element's position deviation.

I set it 20px*20px, this is a empirical value, if i set a value less than it, there  will be problem in ie9.