Have Blink be told of scrollbar changes rather than paying attention itself (Blink side part 2).

Source/web/mac/WebScrollbarTheme.cpp

Index: Source/web/mac/WebScrollbarTheme.cpp
diff --git a/Source/web/mac/WebScrollbarTheme.cpp b/Source/web/mac/WebScrollbarTheme.cpp
index 6692e471c25f9413541d8f66621df9b34995bcc2..69948169629c755482135296ceb94e8b5762139d 100644
--- a/Source/web/mac/WebScrollbarTheme.cpp
+++ b/Source/web/mac/WebScrollbarTheme.cpp
@@ -41,6 +41,10 @@ void WebScrollbarTheme::updateScrollbars(
     float initialButtonDelay, float autoscrollButtonDelay,
     bool jumpOnTrackClick, bool redraw)
 {
+    ScrollbarTheme* theme = ScrollbarTheme::theme();
+    if (theme->isMockTheme())
+        return;
+
     static_cast<ScrollbarThemeMacCommon*>(ScrollbarTheme::theme())->preferencesChanged(
         initialButtonDelay, autoscrollButtonDelay, jumpOnTrackClick, redraw);
 }