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

Source/core/rendering/RenderTheme.cpp

Index: Source/core/rendering/RenderTheme.cpp
diff --git a/Source/core/rendering/RenderTheme.cpp b/Source/core/rendering/RenderTheme.cpp
index a8999b918dcbdda23f16d36ce0915d6be6daca5e..acd3412b5655901d49a35e25fb5afdc01bb41a6b 100644
--- a/Source/core/rendering/RenderTheme.cpp
+++ b/Source/core/rendering/RenderTheme.cpp
@@ -495,6 +495,7 @@ String RenderTheme::extraDefaultStyleSheet()
     if (RuntimeEnabledFeatures::dialogElementEnabled()) {
         runtimeCSS.appendLiteral("dialog:not([open]) { display: none; }");
         runtimeCSS.appendLiteral("dialog { position: absolute; left: 0; right: 0; margin: auto; border: solid; padding: 1em; background: white; color: black;}");
+        runtimeCSS.appendLiteral("dialog::backdrop { background: rgba(0,0,0,0.1); }");
     }
 
     return runtimeCSS.toString();