fix popup position in IE

Test Plan:
In any IE, trigger the login popup-

Busted in prod:
http://fbrell.com/

Fixed after this:
http://fbrell.com/?server=sb

DiffCamp Revision: 117797
Reviewed By: brent
CC: platform-diffs@lists, lshepard, brent, naitik, mattwkelly, selekman
Revert Plan:
OK

src/core/ui.js

@@ -252,10 +252,10 @@ FB.provide('UIServer', {
         : window.screenTop,
       outerWidth = typeof window.outerWidth   != 'undefined'
         ? window.outerWidth
-        : document.body.clientWidth,
+        : document.documentElement.clientWidth,
       outerHeight = typeof window.outerHeight != 'undefined'
         ? window.outerHeight
-        : (document.body.clientHeight - 22),
+        : (document.documentElement.clientHeight - 22), // 22= IE toolbar height
       width    = call.size.width,
       height   = call.size.height,
       left     = parseInt(screenX + ((outerWidth - width) / 2), 10),