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

public/web/mac/WebScrollbarTheme.h

 /*
  * 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 10 matching lines @@
  * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  * 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.
  */
 
-#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
 
 #endif