Add test for paint underinvalidation fix with animated transform overflow.

third_party/WebKit/Source/core/frame/FrameView.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 815 matching lines @@
 
   IntRect documentRect = layoutViewItem.documentRect();
   if (scrollOrigin() == -documentRect.location() &&
       contentsSize() == documentRect.size())
     return;
 
   if (needsLayout())
     return;
 
   // If the visualViewport supplies scrollbars, we won't get a paint
-  // invalidation from computeScrollbarExistence so we need to force one
-  // TODO(bokan): We should avoid computeScrollbarExistence otherwise.
+  // invalidation from computeScrollbarExistence so we need to force one.
   if (visualViewportSuppliesScrollbars())
     layoutViewItem.setMayNeedPaintInvalidation();
 
   // TODO(pdr): This should be refactored to just block scrollbar updates as
   // we are not in a scrollbar update here and m_inUpdateScrollbars has other
   // side effects. This scope is only for preventing a synchronous layout from
   // scroll origin changes which would not be allowed during style recalc.
   InUpdateScrollbarsScope inUpdateScrollbarsScope(this);
 
   bool shouldHaveHorizontalScrollbar = false;
@@ skipping 4355 matching lines @@
 void FrameView::setAnimationHost(
     std::unique_ptr<CompositorAnimationHost> host) {
   m_animationHost = std::move(host);
 }
 
 LayoutUnit FrameView::caretWidth() const {
   return LayoutUnit(getHostWindow()->windowToViewportScalar(1));
 }
 
 }  // namespace blink