RLSA needsLayout on customscrollbar thickness change.

Source/core/rendering/RenderScrollbar.cpp

Index: Source/core/rendering/RenderScrollbar.cpp
diff --git a/Source/core/rendering/RenderScrollbar.cpp b/Source/core/rendering/RenderScrollbar.cpp
index 66577733291e0d8a4059c89c22b96a6c6da36f7e..67647ac3fd26340a632f939b6cab12575dcfb607 100644
--- a/Source/core/rendering/RenderScrollbar.cpp
+++ b/Source/core/rendering/RenderScrollbar.cpp
@@ -204,8 +204,10 @@ void RenderScrollbar::updateScrollbarParts(bool destroy)
 
     if (newThickness != oldThickness) {
         setFrameRect(IntRect(location(), IntSize(isHorizontal ? width() : newThickness, isHorizontal ? newThickness : height())));
-        if (RenderBox* box = owningRenderer())
+        if (RenderBox* box = owningRenderer()) {
+            box->setCustomScrollbarThicknessChanged(true);
             box->setChildNeedsLayout();
+        }
     }
 }