Call ScrollableArea::ShowOverlayScrollbars for explicit scrolls only.

third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  * 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 379 matching lines @@
                                          ScrollbarOrientation orientation) {
   if (ScrollAnimatorBase* scroll_animator = ExistingScrollAnimator()) {
     if (orientation == kVerticalScrollbar)
       scroll_animator->WillRemoveVerticalScrollbar(scrollbar);
     else
       scroll_animator->WillRemoveHorizontalScrollbar(scrollbar);
   }
 }
 
 void ScrollableArea::ContentsResized() {
-  ShowOverlayScrollbars();
   if (ScrollAnimatorBase* scroll_animator = ExistingScrollAnimator())
     scroll_animator->ContentsResized();
 }
 
 bool ScrollableArea::HasOverlayScrollbars() const {
   Scrollbar* v_scrollbar = VerticalScrollbar();
   if (v_scrollbar && v_scrollbar->IsOverlayScrollbar())
     return true;
   Scrollbar* h_scrollbar = HorizontalScrollbar();
   return h_scrollbar && h_scrollbar->IsOverlayScrollbar();
@@ skipping 260 matching lines @@
                                          offset.y() - ScrollOrigin().Y());
   SetScrollOffset(new_offset, kCompositorScroll);
 }
 
 DEFINE_TRACE(ScrollableArea) {
   visitor->Trace(scroll_animator_);
   visitor->Trace(programmatic_scroll_animator_);
 }
 
 }  // namespace blink