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/WebDOMProgressEvent.cpp b/Source/web/mac/WebScrollbarTheme.cpp
similarity index 80%
copy from Source/web/WebDOMProgressEvent.cpp
copy to Source/web/mac/WebScrollbarTheme.cpp
index a588c106c675509baba4618ce2dbea715aaa99bc..050f1c22c0e57402e36edf1a9c584fa9a181db59 100644
--- a/Source/web/WebDOMProgressEvent.cpp
+++ b/Source/web/mac/WebScrollbarTheme.cpp
@@ -29,20 +29,22 @@
  */
 
 #include "config.h"
-#include "WebDOMProgressEvent.h"
+#include "WebScrollbarTheme.h"
 
-#include "core/events/ProgressEvent.h"
-#include "public/platform/WebString.h"
+#include "core/platform/ScrollbarThemeMacCommon.h"
 
 using namespace WebCore;
 
 namespace WebKit {
 
-WebDOMProgressEvent::WebDOMProgressEvent(const WebString& type, bool lengthIsComputable, unsigned long long loaded, unsigned long long total)
-    : WebDOMEvent(ProgressEvent::create(type, lengthIsComputable, loaded, total))
+#if OS(MACOSX)
+void WebScrollbarTheme::updateScrollbars(
+    float initialButtonDelay, float autoscrollButtonDelay,
+    bool jumpOnTrackClick, bool redraw)
 {
-    ASSERT(m_private.get());
-    ASSERT(isProgressEvent());
+    static_cast<ScrollbarThemeMacCommon*>(ScrollbarTheme::theme())->preferencesChanged(
+        initialButtonDelay, autoscrollButtonDelay, jumpOnTrackClick, redraw);
 }
+#endif
 
 } // namespace WebKit