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

Source/core/rendering/style/RenderStyle.h

Index: Source/core/rendering/style/RenderStyle.h
diff --git a/Source/core/rendering/style/RenderStyle.h b/Source/core/rendering/style/RenderStyle.h
index ebdd80e54b4d7de2e8056366000d6ccc56b59c5b..0a016689f7ab281c08e3928cebc852e3c63351b8 100644
--- a/Source/core/rendering/style/RenderStyle.h
+++ b/Source/core/rendering/style/RenderStyle.h
@@ -241,7 +241,7 @@ protected:
         unsigned _page_break_inside : 2; // EPageBreak
 
         unsigned _styleType : 6; // PseudoId
-        unsigned _pseudoBits : 7;
+        unsigned _pseudoBits : 8;
         unsigned explicitInheritance : 1; // Explicitly inherits a non-inherited property
         unsigned unique : 1; // Style can not be shared.
         unsigned emptyState : 1;
@@ -262,7 +262,7 @@ protected:
         unsigned _affectedByDrag : 1;
         unsigned _isLink : 1;
         // If you add more style bits here, you will also need to update RenderStyle::copyNonInheritedFrom()
-        // 59 bits
+        // 60 bits
     } noninherited_flags;
 
 // !END SYNC!