Add test for paint underinvalidation fix with animated transform overflow.

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

Index: third_party/WebKit/Source/core/frame/FrameView.cpp
diff --git a/third_party/WebKit/Source/core/frame/FrameView.cpp b/third_party/WebKit/Source/core/frame/FrameView.cpp
index 333d17dea30e5a45a176685a846d9fb2bab5e21e..e20993a6548a41c1782c27cea5084c27f6d933ad 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -833,8 +833,7 @@ void FrameView::recalcOverflowAfterStyleChange() {
     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();