Feed ScrollableArea::showOverlayScrollbars into ScrollbarAnimationController.

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp

Index: third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
index 77a1c5238ab467d5621978ae2db2c9e4e4dad0f2..0d7a5af2efe5d12edeefffe5439c89ca4c5a8010 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -1936,6 +1936,12 @@ bool CompositedLayerMapping::updateOverflowControlsLayers(
       toggleScrollbarLayerIfNeeded(m_layerForVerticalScrollbar, false,
                                    CompositingReasonLayerForVerticalScrollbar);
     scrollableArea->resetRebuildScrollbarLayerFlags();
+
+    if (m_scrollingContentsLayer &&
+        scrollableArea->needsShowScrollbarLayers()) {
+      m_scrollingContentsLayer->platformLayer()->showScrollbars();
+      scrollableArea->didShowScrollbarLayers();
+    }
   }
 
   // If the subtree is invisible, we don't actually need scrollbar layers.