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

Source/core/rendering/style/RenderStyleConstants.h

Index: Source/core/rendering/style/RenderStyleConstants.h
diff --git a/Source/core/rendering/style/RenderStyleConstants.h b/Source/core/rendering/style/RenderStyleConstants.h
index 7dd5c70ee9db028044307c92c9d2ff89b8d4adf8..54c4c594e3de8905d98d8588b3011fc1cad95e0c 100644
--- a/Source/core/rendering/style/RenderStyleConstants.h
+++ b/Source/core/rendering/style/RenderStyleConstants.h
@@ -70,7 +70,8 @@ enum StyleDifferenceContextSensitiveProperty {
 // Static pseudo styles. Dynamic ones are produced on the fly.
 enum PseudoId {
     // The order must be NOP ID, public IDs, and then internal IDs.
-    NOPSEUDO, FIRST_LINE, FIRST_LETTER, BEFORE, AFTER, SELECTION, FIRST_LINE_INHERITED, SCROLLBAR,
+    // If you add or remove a public ID, you must update _pseudoBits in RenderStyle.
+    NOPSEUDO, FIRST_LINE, FIRST_LETTER, BEFORE, AFTER, BACKDROP, SELECTION, FIRST_LINE_INHERITED, SCROLLBAR,
     // Internal IDs follow:
     SCROLLBAR_THUMB, SCROLLBAR_BUTTON, SCROLLBAR_TRACK, SCROLLBAR_TRACK_PIECE, SCROLLBAR_CORNER, RESIZER,
     INPUT_LIST_BUTTON,