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

Source/core/css/resolver/StyleResolver.cpp

Index: Source/core/css/resolver/StyleResolver.cpp
diff --git a/Source/core/css/resolver/StyleResolver.cpp b/Source/core/css/resolver/StyleResolver.cpp
index 01daeb9f6769590ad0a17cdb7e5962f66a24cf63..2fd0d067d8435e4d79ce09754932eb72ff5c219e 100644
--- a/Source/core/css/resolver/StyleResolver.cpp
+++ b/Source/core/css/resolver/StyleResolver.cpp
@@ -1246,7 +1246,7 @@ PassRefPtr<RenderStyle> StyleResolver::pseudoStyleForElement(Element* e, const P
 
     state.initForStyleResolve(document(), e, parentStyle);
 
-    if (m_state.parentStyle()) {
+    if (pseudoStyleRequest.pseudoId != BACKDROP && m_state.parentStyle()) { // The spec disallows inheritance for ::backdrop.
         state.setStyle(RenderStyle::create());
         state.style()->inheritFrom(m_state.parentStyle());
     } else {
@@ -1495,8 +1495,10 @@ void StyleResolver::adjustRenderStyle(RenderStyle* style, RenderStyle* parentSty
             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))
@@ -1551,7 +1553,7 @@ void StyleResolver::adjustRenderStyle(RenderStyle* style, RenderStyle* parentSty
         || style->hasBlendMode()
         || style->position() == StickyPosition
         || (style->position() == FixedPosition && e && e->document()->page() && e->document()->page()->settings()->fixedPositionCreatesStackingContext())
-        || (e && e->isInTopLayer())
+        || ((e && e->isInTopLayer()) || style->styleType() == BACKDROP)

[Julien - ping for review, 2013/07/03 01:48:05]

This is now repeated twice, which means a helper function would be nice.

[falken, 2013/07/03 06:14:48]

Done.

         ))
         style->setZIndex(0);