[cssom] Proposing a getDefaultComputedStyle method

Proposal: a getDefaultComputedStyle method that acts just like 
getComputedStyle but returns what the style would be if it were not for 
author rules.

Rationale: jQuery has hacks to try to figure out default styles, 
involving creation of subframes and so forth.  These hacks are very 
fragile.  There's got to be a better way for libraries to do this sort 
of thing.

IDL:

     CSSStyleDeclaration
       getDefaultComputedStyle(Element elt,
                               optional DOMString pseudoElt);

Behavior is just like getComputedStyle except:

1) Ignores all author-level sheets, transitions, animations,
    override stylesheets.
2) Never returns used styles, only computed ones (just like
    getComputedStyle would for a display:none element).

My implementation at the moment also ignores presentational hints, but 
I'm open to changing that if people think it would be better to include 
them.

See also https://bugzilla.mozilla.org/show_bug.cgi?id=800983 for the 
implementation and https://bugzilla.mozilla.org/show_bug.cgi?id=797029 
for the jQuery issues that prompted this suggestion.  The jQuery folks 
sound like they would really like something like this, for what it's worth.

-Boris