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

Source/core/platform/ScrollbarThemeMacCommon.h

 /*
  * Copyright (C) 2008 Apple 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 19 matching lines @@
 
 namespace WebCore {
 
 class ScrollbarThemeMacCommon : public ScrollbarTheme {
 public:
     ScrollbarThemeMacCommon();
     virtual ~ScrollbarThemeMacCommon();
 
     virtual void registerScrollbar(ScrollbarThemeClient*) OVERRIDE;
     virtual void unregisterScrollbar(ScrollbarThemeClient*) OVERRIDE;
-    void preferencesChanged();
+    void preferencesChanged(float, float, bool, bool);

[abarth-chromium, 2013/11/04 00:43:24]

Would you be willing to name these parameters?  It's not clear what they
represent.

[Avi (use Gerrit), 2013/11/04 01:40:24]

Of course I'm willing. I'm writing to fit in with the existing style, but if you
ask for it, you get it.

[abarth-chromium, 2013/11/04 01:41:55]

The existing style is to omit parameter names when they don't add any additional
information.  For example, naming the GraphicsContext* parameters below
|context| wouldn't convey any additional information.

 
     virtual bool supportsControlTints() const OVERRIDE { return true; }
 
     virtual double initialAutoscrollTimerDelay() OVERRIDE;
     virtual double autoscrollTimerDelay() OVERRIDE;
 
     virtual void paintOverhangBackground(GraphicsContext*, const IntRect& horizontalOverhangArea, const IntRect& verticalOverhangArea, const IntRect& dirtyRect) OVERRIDE;
     virtual void paintOverhangShadows(GraphicsContext*, const IntSize& scrollOffset, const IntRect& horizontalOverhangArea, const IntRect& verticalOverhangArea, const IntRect& dirtyRect) OVERRIDE;
     virtual void paintTickmarks(GraphicsContext*, ScrollbarThemeClient*, const IntRect&) OVERRIDE;
 
 protected:
     virtual int maxOverlapBetweenPages() OVERRIDE { return 40; }
 
     virtual bool shouldCenterOnThumb(ScrollbarThemeClient*, const PlatformMouseEvent&) OVERRIDE;
     virtual bool shouldDragDocumentInsteadOfThumb(ScrollbarThemeClient*, const PlatformMouseEvent&) OVERRIDE;
     int scrollbarPartToHIPressedState(ScrollbarPart);
 
     virtual void updateButtonPlacement() { }
 
     void paintGivenTickmarks(GraphicsContext*, ScrollbarThemeClient*, const IntRect&, const Vector<IntRect>&);
 
     RefPtr<Pattern> m_overhangPattern;
 };
 
 }
 
 #endif // ScrollbarThemeMacCommon_h