Add a use counter of css clip of fixed-pos elements.

third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
index 7f61d9b1baadc659030c4925fcd1398abedd5592..615790ee60e6acf14385a85f10807f5a290109b5 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
@@ -501,6 +501,8 @@ PaintLayerPainter::PaintResult PaintLayerPainter::paintLayerWithTransform(Graphi
             if (needsToClip(paintingInfo, clipRectForFragment)) {
                 if (m_paintLayer.layoutObject()->isPositioned() && clipRectForFragment.isClippedByClipCss())
                     UseCounter::count(m_paintLayer.layoutObject()->document(), UseCounter::ClipCssOfPositionedElement);
+                if (m_paintLayer.layoutObject()->isFixedPositioned())
+                    UseCounter::count(m_paintLayer.layoutObject()->document(), UseCounter::ClipCssOfFixedPositionElement);
                 clipRecorder.emplace(context, *parentLayer->layoutObject(), DisplayItem::kClipLayerParent, clipRectForFragment, &paintingInfo, fragment.paginationOffset, paintFlags);
             }
         }