CSS: Do not throw on frame elements in FF

IE9-10 throws on elements created in popups (see #14150), FF meanwhile throws
on frame elements through "defaultView.getComputedStyle" (see #15098)

Use "defaultView" if in the popup which would fix IE issue,
use "window.getComputedStyle" which would fix FF issue.

And everybody wins, except performance, but who cares right?

Fixes #15098