CSS: Do not throw on frame elements in FF

IE9-10 throws on elements created in popups (see <a href="http://bugs.jquery.com/ticket/14150">#14150</a>), FF meanwhile throws on frame elements through "defaultView.getComputedStyle" (see <a href="http://bugs.jquery.com/ticket/15098">#15098</a>)

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