Repaint overlay scrollbar if resource is disposed.

cc/layers/painted_overlay_scrollbar_layer.cc

Index: cc/layers/painted_overlay_scrollbar_layer.cc
diff --git a/cc/layers/painted_overlay_scrollbar_layer.cc b/cc/layers/painted_overlay_scrollbar_layer.cc
index df323479d491beefb789fadc7e550a36b538f2b6..81a515edf1cefd80fa6456b997ac642adaaefe68 100644
--- a/cc/layers/painted_overlay_scrollbar_layer.cc
+++ b/cc/layers/painted_overlay_scrollbar_layer.cc
@@ -138,7 +138,7 @@ bool PaintedOverlayScrollbarLayer::Update() {
 }
 
 bool PaintedOverlayScrollbarLayer::PaintThumbIfNeeded() {
-  if (!scrollbar_->NeedsPaintPart(THUMB))
+  if (!scrollbar_->NeedsPaintPart(THUMB) && thumb_resource_)
     return false;
 
   gfx::Rect paint_rect = OriginThumbRectForPainting();