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

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

Index: third_party/WebKit/Source/core/layout/compositing/CompositingInputsUpdater.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositingInputsUpdater.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositingInputsUpdater.cpp
index d3889b9c879c8ef9142a429764b90dfcc6537d1e..c8f3f1db5a3cff773419d8e7fc1d10631c5cee93 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositingInputsUpdater.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositingInputsUpdater.cpp
@@ -186,6 +186,13 @@ void CompositingInputsUpdater::UpdateRecursive(PaintLayer* layer,
                                        : 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();