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 d5d2ada34acdaf02030d9a5807a9c7320d153e04..40ecc5d108921bc822536c8332b6e32fbe0b5248 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
@@ -486,6 +486,8 @@ PaintLayerPainter::PaintResult PaintLayerPainter::paintLayerWithTransform(Graphi
                 if (m_paintLayer.layoutObject()->isPositioned() && clipRectForFragment.isClippedByClipCss())
                     UseCounter::count(m_paintLayer.layoutObject()->document(), UseCounter::ClipCssOfPositionedElement);
                 clipRecorder.emplace(context, *parentLayer->layoutObject(), DisplayItem::ClipLayerParent, clipRectForFragment, &paintingInfo, fragment.paginationOffset, paintFlags);
+                if (m_paintLayer.layoutObject()->isFixedPositioned())
+                    UseCounter::count(m_paintLayer.layoutObject()->document(), UseCounter::ClipCssOfFixedPositionElement);
             }
         }
         if (paintFragmentByApplyingTransform(context, paintingInfo, paintFlags, fragment.paginationOffset) == MayBeClippedByPaintDirtyRect)