Remove the concept of document.documentElement

sky/engine/core/frame/LocalDOMWindow.cpp

Index: sky/engine/core/frame/LocalDOMWindow.cpp
diff --git a/sky/engine/core/frame/LocalDOMWindow.cpp b/sky/engine/core/frame/LocalDOMWindow.cpp
index fc35a2003f430950614eaddc34d6aa7e3afe73e9..8960f7ba1bb724df6523e8d1b3a64a84e843e016 100644
--- a/sky/engine/core/frame/LocalDOMWindow.cpp
+++ b/sky/engine/core/frame/LocalDOMWindow.cpp
@@ -69,6 +69,7 @@
 #include "sky/engine/platform/weborigin/KURL.h"
 #include "sky/engine/platform/weborigin/SecurityPolicy.h"
 #include "sky/engine/public/platform/Platform.h"
+#include "sky/engine/tonic/dart_gc_visitor.h"
 #include "sky/engine/wtf/MainThread.h"
 #include "sky/engine/wtf/MathExtras.h"
 #include "sky/engine/wtf/text/WTFString.h"
@@ -310,6 +311,11 @@ LocalDOMWindow* LocalDOMWindow::toDOMWindow()
     return this;
 }
 
+void LocalDOMWindow::AcceptDartGCVisitor(DartGCVisitor& visitor) const {
+    visitor.AddToSetForRoot(document(), dart_wrapper());
+    EventTarget::AcceptDartGCVisitor(visitor);
+}
+
 PassRefPtr<MediaQueryList> LocalDOMWindow::matchMedia(const String& media)
 {
     return document() ? document()->mediaQueryMatcher().matchMedia(media) : nullptr;