Add a use-counter for viewport-fixed position:fixed under a filter

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

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/layout/compositing/CompositingInputsUpdater.h"
 
 #include "core/dom/Document.h"
 #include "core/frame/FrameView.h"
 #include "core/layout/LayoutBlock.h"
 #include "core/layout/LayoutView.h"
@@ skipping 168 matching lines @@
       const PaintLayer* parent = layer->Parent();
       properties.opacity_ancestor =
           parent->IsTransparent() ? parent : parent->OpacityAncestor();
       properties.transform_ancestor =
           parent->Transform() ? parent : parent->TransformAncestor();
       properties.filter_ancestor = parent->HasFilterInducingProperty()
                                        ? parent
                                        : parent->FilterAncestor();
       bool layer_is_fixed_position =
           layer->GetLayoutObject().Style()->GetPosition() == EPosition::kFixed;
+
+      if (layer_is_fixed_position && properties.filter_ancestor &&
+          layer->FixedToViewport()) {
+        UseCounter::Count(layer->GetLayoutObject().GetDocument(),
+                          UseCounter::kViewportFixedPositionUnderFilter);
+      }
+
       properties.nearest_fixed_position_layer =
           layer_is_fixed_position ? layer : parent->NearestFixedPositionLayer();
 
       if (info.has_ancestor_with_clip_related_property) {
         const PaintLayer* parent_layer_on_clipping_container_chain =
             FindParentLayerOnClippingContainerChain(layer);
         const bool parent_has_clip_related_property =
             parent_layer_on_clipping_container_chain->GetLayoutObject()
                 .HasClipRelatedProperty();
         properties.clipping_container =
@@ skipping 79 matching lines @@
   DCHECK(!layer->NeedsCompositingInputsUpdate());
 
   for (PaintLayer* child = layer->FirstChild(); child;
        child = child->NextSibling())
     AssertNeedsCompositingInputsUpdateBitsCleared(child);
 }
 
 #endif
 
 }  // namespace blink