Make dialog::backdrop span the viewport by default.

LayoutTests/fast/dom/HTMLDialogElement/modal-dialog-blocks-mouse-events.html

Index: LayoutTests/fast/dom/HTMLDialogElement/modal-dialog-blocks-mouse-events.html
diff --git a/LayoutTests/fast/dom/HTMLDialogElement/modal-dialog-blocks-mouse-events.html b/LayoutTests/fast/dom/HTMLDialogElement/modal-dialog-blocks-mouse-events.html
index 43720da1a236a0b6fcb8630b6644f13ae63fedde..1d33d7ed8b3c123ee04a6feb5f6d659e615ddf80 100644
--- a/LayoutTests/fast/dom/HTMLDialogElement/modal-dialog-blocks-mouse-events.html
+++ b/LayoutTests/fast/dom/HTMLDialogElement/modal-dialog-blocks-mouse-events.html
@@ -12,6 +12,10 @@ dialog {
     width: 100px;
 }
 
+dialog::backdrop {
+    display: none;
+}
+
 #dialog-div {
     height: 100px;
     width: 100px;