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 6b7e2e1d73d0f7bc42cbace38f311f3d8af8d2db..d87f45a20c47042df11345364da75b8eb878692c 100644
--- a/Source/core/css/resolver/StyleResolver.cpp
+++ b/Source/core/css/resolver/StyleResolver.cpp
@@ -1245,7 +1245,7 @@ PassRefPtr<RenderStyle> StyleResolver::pseudoStyleForElement(Element* e, const P
 
     state.initForStyleResolve(document(), e, parentStyle);
 
-    if (m_state.parentStyle()) {
+    if (pseudoStyleRequest.pseudoId != BACKDROP && m_state.parentStyle()) {

[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 {
@@ -1494,8 +1494,10 @@ 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))