Set <dialog> width and height to 'fit-content'.

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 c73b9834c180554a188f9485d252975f0ae0b614..f7a1be695bc56083ee7d32521a6ba682acd2509b 100644
--- a/Source/core/rendering/RenderTheme.cpp
+++ b/Source/core/rendering/RenderTheme.cpp
@@ -497,7 +497,7 @@ 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 { position: absolute; left: 0; right: 0; width: -webkit-fit-content; height: -webkit-fit-content; margin: auto; border: solid; padding: 1em; background: white; color: black;}");
         runtimeCSS.appendLiteral("dialog::backdrop { background: rgba(0,0,0,0.1); }");
     }