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

public/web/mac/WebScrollbarTheme.h

Index: public/web/mac/WebScrollbarTheme.h
diff --git a/public/platform/WebSourceBuffer.h b/public/web/mac/WebScrollbarTheme.h
similarity index 69%
copy from public/platform/WebSourceBuffer.h
copy to public/web/mac/WebScrollbarTheme.h
index f2cd1954bf9f352080b88fcda59e8d35474915c1..48a2afccd4f74361db021166c6aeb115284546cb 100644
--- a/public/platform/WebSourceBuffer.h
+++ b/public/web/mac/WebScrollbarTheme.h
@@ -28,30 +28,23 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#ifndef WebSourceBuffer_h
-#define WebSourceBuffer_h
+#ifndef WebScrollbarTheme_h
+#define WebScrollbarTheme_h
 
-#include "WebTimeRange.h"
+#include "public/platform/WebCommon.h"
 
 namespace WebKit {
 
-class WebSourceBuffer {
+class WebScrollbarTheme {
 public:
-    virtual ~WebSourceBuffer() { }
-
-    virtual WebTimeRanges buffered() = 0;
-    virtual void append(const unsigned char* data, unsigned length) = 0;
-    virtual void abort() = 0;
-    virtual void remove(double start, double end) = 0;
-    virtual bool setTimestampOffset(double) = 0;
-
-    // Set presentation timestamp for the start of append window.
-    virtual void setAppendWindowStart(double) = 0;
-
-    // Set presentation timestamp for the end of append window.
-    virtual void setAppendWindowEnd(double) = 0;
-
-    virtual void removedFromMediaSource() = 0;
+    // Updates the scrollbar appearance and behavior.
+    // |initialButtonDelay| is the current value of NSScrollerButtonDelay from NSUserDefaults.
+    // |autoscrollButtonDelay| is the current value of NSScrollerButtonPeriod from NSUserDefaults.
+    // |jumpOnTrackClick| is the current value of AppleScrollerPagingBehavior from NSUserDefaults.
+    // |redraw| is true if the update requires a redraw to include the change.
+    BLINK_EXPORT static void updateScrollbars(
+        float initialButtonDelay, float autoscrollButtonDelay,
+        bool jumpOnTrackClick, bool redraw);
 };
 
 } // namespace WebKit