Modify the UA stylesheet to allow <dialog popup>

This CL does two main things:
 1. Convert the :-internal-popup-hidden to
    :-internal-popup-opening-or-open
 2. Add/modify rules for both dialog and [popup] so that they don't
    add display:none when a <dialog popup> is either open as a
    dialog or pop-up.

See this issue for details:
   openui/open-ui#520

Bug: 1307772
Change-Id: I751f6991a988a58032cebd52f289e5468140ce79

html/semantics/popups/popup-attribute-basic.tentative.html

@@ -17,13 +17,16 @@
   <header popup>Different element type</header>
   <nav popup>Different element type</nav>
   <input type=text popup value="Different element type">
+  <dialog popup>Dialog with popup attribute</dialog>
+  <dialog popup="manual">Dialog with popup=manual</dialog>
   <div popup=true>Invalid popup value - defaults to popup=manual</div>
   <div popup=popup>Invalid popup value - defaults to popup=manual</div>
   <div popup=invalid>Invalid popup value - defaults to popup=manual</div>
 </div>
 
 <div id=nonpopups>
   <div>Not a pop-up</div>
+  <dialog open>Dialog without popup attribute</dialog>
 </div>
 
 <div popup class=animated>Animated pop-up</div>