Simplification of css_defaultDisplay helper (fix #12723)

1) Fix #12723: use getDefaultComputedStyle method in FF to circumvent iframe path
2) Rename helper "css_defaultDisplay" to "defaultDisplay" since it used only in css module 3) Take away fix for #10227 ticket, as it is no longer needed.
4) Improve test for #10227 fix - iframe of defaultDisplay helper appended to html element,
    so check correct default display for html element too
5) Simplify iframe creation

/cc @mikesherov, @rwldrn