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

Source/web/mac/WebScrollbarTheme.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 11 matching lines @@
  * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #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