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

Source/core/platform/ScrollbarThemeMacCommon.mm

 /*
  * Copyright (C) 2008, 2011 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 72 matching lines @@
 @implementation WebScrollbarPrefsObserver
 
 + (void)appearancePrefsChanged:(NSNotification*)unusedNotification
 {
     UNUSED_PARAM(unusedNotification);
 
     ScrollbarTheme* theme = ScrollbarTheme::theme();
     if (theme->isMockTheme())
         return;
 
-    static_cast<ScrollbarThemeMacCommon*>(ScrollbarTheme::theme())->preferencesChanged();
-    if (scrollbarSet().isEmpty())
-        return;
-    ScrollbarSet::iterator end = scrollbarSet().end();
-    for (ScrollbarSet::iterator it = scrollbarSet().begin(); it != end; ++it) {
-        (*it)->styleChanged();
-        (*it)->invalidate();
-    }
+    NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
+    [defaults synchronize];
+    static_cast<ScrollbarThemeMacCommon*>(ScrollbarTheme::theme())->preferencesChanged(
+        [defaults floatForKey:@"NSScrollerButtonDelay"], [defaults floatForKey:@"NSScrollerButtonPeriod"],
+        [defaults boolForKey:@"AppleScrollerPagingBehavior"], true);
 }
 
 + (void)behaviorPrefsChanged:(NSNotification*)unusedNotification
 {
     UNUSED_PARAM(unusedNotification);
 
     ScrollbarTheme* theme = ScrollbarTheme::theme();
     if (theme->isMockTheme())
         return;
 
-    static_cast<ScrollbarThemeMacCommon*>(ScrollbarTheme::theme())->preferencesChanged();
+    NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
+    [defaults synchronize];
+    static_cast<ScrollbarThemeMacCommon*>(ScrollbarTheme::theme())->preferencesChanged(
+        [defaults floatForKey:@"NSScrollerButtonDelay"], [defaults floatForKey:@"NSScrollerButtonPeriod"],
+        [defaults boolForKey:@"AppleScrollerPagingBehavior"], false);
 }
 
 + (void)registerAsObserver
 {
     [[NSDistributedNotificationCenter defaultCenter] addObserver:self selector:@selector(appearancePrefsChanged:) name:@"AppleAquaScrollBarVariantChanged" object:nil suspensionBehavior:NSNotificationSuspensionBehaviorDeliverImmediately];
     [[NSDistributedNotificationCenter defaultCenter] addObserver:self selector:@selector(behaviorPrefsChanged:) name:@"AppleNoRedisplayAppearancePreferenceChanged" object:nil suspensionBehavior:NSNotificationSuspensionBehaviorCoalesce];
 }
 
 @end
 
@@ skipping 211 matching lines @@
     tickmarkTrackRect.setWidth(tickmarkTrackRect.width() - 2);
     paintGivenTickmarks(context, scrollbar, tickmarkTrackRect, tickmarks);
 }
 
 ScrollbarThemeMacCommon::ScrollbarThemeMacCommon()
 {
     static bool initialized;
     if (!initialized) {
         initialized = true;
         [WebScrollbarPrefsObserver registerAsObserver];
-        preferencesChanged();
+        NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
+        [defaults synchronize];
+        preferencesChanged(
+            [defaults floatForKey:@"NSScrollerButtonDelay"], [defaults floatForKey:@"NSScrollerButtonPeriod"],
+            [defaults boolForKey:@"AppleScrollerPagingBehavior"], false);
     }
 }
 
 ScrollbarThemeMacCommon::~ScrollbarThemeMacCommon()
 {
 }
 
-void ScrollbarThemeMacCommon::preferencesChanged()
+void ScrollbarThemeMacCommon::preferencesChanged(float initialButtonDelay, float autoscrollButtonDelay, bool jumpOnTrackClick, bool redraw)
 {
-    NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
-    [defaults synchronize];
     updateButtonPlacement();
-    gInitialButtonDelay = [defaults floatForKey:@"NSScrollerButtonDelay"];
-    gAutoscrollButtonDelay = [defaults floatForKey:@"NSScrollerButtonPeriod"];
-    gJumpOnTrackClick = [defaults boolForKey:@"AppleScrollerPagingBehavior"];
+    gInitialButtonDelay = initialButtonDelay;
+    gAutoscrollButtonDelay = autoscrollButtonDelay;
+    gJumpOnTrackClick = jumpOnTrackClick;
+    if (redraw && !scrollbarSet().isEmpty()) {
+        ScrollbarSet::iterator end = scrollbarSet().end();
+        for (ScrollbarSet::iterator it = scrollbarSet().begin(); it != end; ++it) {
+            (*it)->styleChanged();
+            (*it)->invalidate();
+        }
+    }
 }
 
 double ScrollbarThemeMacCommon::initialAutoscrollTimerDelay()
 {
     return gInitialButtonDelay;
 }
 
 double ScrollbarThemeMacCommon::autoscrollTimerDelay()
 {
     return gAutoscrollButtonDelay;
@@ skipping 25 matching lines @@
         case ForwardButtonEndPart:
             return kThemeBottomOutsideArrowPressed;
         case ThumbPart:
             return kThemeThumbPressed;
         default:
             return 0;
     }
 }
 
 } // namespace WebCore