Have Blink be told of scrollbar changes rather than paying attention itself (Blink side).

Source/web/mac/WebScrollbarTheme.cpp

Index: Source/web/mac/WebScrollbarTheme.cpp
diff --git a/Source/web/WebCustomElement.cpp b/Source/web/mac/WebScrollbarTheme.cpp
similarity index 81%
copy from Source/web/WebCustomElement.cpp
copy to Source/web/mac/WebScrollbarTheme.cpp
index f34222e06d6618038524382490e65e9d7e6fa746..d242f31c6ac2ca81986f149a14f60d427f0a31a0 100644
--- a/Source/web/WebCustomElement.cpp
+++ b/Source/web/mac/WebScrollbarTheme.cpp
@@ -29,20 +29,20 @@
  */
 
 #include "config.h"
-#include "WebCustomElement.h"
+#include "WebScrollbarTheme.h"
 
-#include "../platform/WebString.h"
-#include "RuntimeEnabledFeatures.h"
-#include "core/dom/custom/CustomElement.h"
+#include "core/platform/ScrollbarThemeMacCommon.h"
 
 using namespace WebCore;
 
 namespace WebKit {
 
-void WebCustomElement::addEmbedderCustomElementName(const WebString& name)
+void WebScrollbarTheme::updateScrollbars(
+    float initialButtonDelay, float autoscrollButtonDelay,
+    bool jumpOnTrackClick, bool redraw)
 {
-    ASSERT(RuntimeEnabledFeatures::embedderCustomElementsEnabled());
-    CustomElement::addEmbedderCustomElementName(name);
+    static_cast<ScrollbarThemeMacCommon*>(ScrollbarTheme::theme())->preferencesChanged(
+        initialButtonDelay, autoscrollButtonDelay, jumpOnTrackClick, redraw);
 }
 
 } // namespace WebKit