Implement the ::backdrop pseudo-element for modal <dialog>.

Source/core/css/resolver/StyleResolver.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (c) 2011, Code Aurora Forum. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
@@ skipping 1227 matching lines @@
     ASSERT(parentStyle);
     if (!e)
         return 0;
 
     StyleResolverState& state = m_state;
 
     initElement(e);
 
     state.initForStyleResolve(document(), e, parentStyle);
 
-    if (m_state.parentStyle()) {
+    if (pseudoStyleRequest.pseudoId != BACKDROP && m_state.parentStyle()) {

[Julien - ping for review, 2013/06/27 22:49:00]

It's weird to disallow style cascading for ::backdrop.

[falken, 2013/06/28 08:06:52]

The spec says ::backdrop "does not inherit from any element
and is not inherited from."
This check should prevent ::backdrop from inheriting from
anything. I think ::backdrop will never be a parent, so
nothing can inherit from it.

It looks like cascading still works fine even with the check.
E.g., the UA stylesheet for ::backdrop background is
overridden in many of my tests.

I've added a comment to the code.

         state.setStyle(RenderStyle::create());
         state.style()->inheritFrom(m_state.parentStyle());
     } else {
         state.setStyle(defaultStyleForElement());
         state.setParentStyle(RenderStyle::clone(state.style()));
     }
 
     // Since we don't use pseudo-elements in any of our quirk/print user agent rules, don't waste time walking
     // those rules.
 
@@ skipping 228 matching lines @@
 
         // FIXME: We shouldn't be overriding start/-webkit-auto like this. Do it in html.css instead.
         // Table headers with a text-align of -webkit-auto will change the text-align to center.
         if (e && e->hasTagName(thTag) && style->textAlign() == TASTART)
             style->setTextAlign(CENTER);
 
         if (e && e->hasTagName(legendTag))
             style->setDisplay(BLOCK);
 
         // Per the spec, position 'static' and 'relative' in the top layer compute to 'absolute'.
-        if (e && e->isInTopLayer() && (style->position() == StaticPosition || style->position() == RelativePosition))
-            style->setPosition(AbsolutePosition);
+        if ((e && e->isInTopLayer()) || style->styleType() == BACKDROP) {
+            if (style->position() == StaticPosition || style->position() == RelativePosition)
+                style->setPosition(AbsolutePosition);
+        }
 
         // Absolute/fixed positioned elements, floating elements and the document element need block-like outside display.
         if (style->hasOutOfFlowPosition() || style->isFloating() || (e && e->document()->documentElement() == e))
             style->setDisplay(equivalentBlockDisplay(style->display(), style->isFloating(), !document()->inQuirksMode()));
 
         // FIXME: Don't support this mutation for pseudo styles like first-letter or first-line, since it's not completely
         // clear how that should work.
         if (style->display() == INLINE && style->styleType() == NOPSEUDO && style->writingMode() != parentStyle->writingMode())
             style->setDisplay(INLINE_BLOCK);
 
@@ skipping 2176 matching lines @@
     info.addMember(m_state, "state");
 
     // FIXME: move this to a place where it would be called only once?
     info.addMember(CSSDefaultStyleSheets::defaultStyle, "defaultStyle");
     info.addMember(CSSDefaultStyleSheets::defaultQuirksStyle, "defaultQuirksStyle");
     info.addMember(CSSDefaultStyleSheets::defaultPrintStyle, "defaultPrintStyle");
     info.addMember(CSSDefaultStyleSheets::defaultViewSourceStyle, "defaultViewSourceStyle");
 }
 
 } // namespace WebCore