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

third_party/WebKit/Source/core/paint/PaintLayerPainter.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/paint/PaintLayerPainter.h"
 
 #include "core/frame/LocalFrame.h"
 #include "core/layout/LayoutInline.h"
 #include "core/layout/LayoutView.h"
 #include "core/paint/ClipPathClipper.h"
@@ skipping 483 matching lines @@
             // A fixed-position object is repeated on every page, but if it is clipped by an ancestor layer then
             // the repetitions are clipped out.
             if (!isFixedPosObjectInPagedMedia)
                 clipRectForFragment.moveBy(fragment.paginationOffset);
             clipRectForFragment.intersect(fragment.backgroundRect);
             if (clipRectForFragment.isEmpty())
                 continue;
             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);
             }
         }
         if (paintFragmentByApplyingTransform(context, paintingInfo, paintFlags, fragment.paginationOffset) == MayBeClippedByPaintDirtyRect)
             result = MayBeClippedByPaintDirtyRect;
     }
     return result;
 }
 
 PaintLayerPainter::PaintResult PaintLayerPainter::paintFragmentByApplyingTransform(GraphicsContext& context, const PaintLayerPaintingInfo& paintingInfo, PaintLayerFlags paintFlags, const LayoutPoint& fragmentTranslation)
@@ skipping 276 matching lines @@
     if (!m_paintLayer.containsDirtyOverlayScrollbars())
         return;
 
     PaintLayerPaintingInfo paintingInfo(&m_paintLayer, LayoutRect(enclosingIntRect(damageRect)), paintFlags, LayoutSize());
     paintLayer(context, paintingInfo, PaintLayerPaintingOverlayScrollbars);
 
     m_paintLayer.setContainsDirtyOverlayScrollbars(false);
 }
 
 } // namespace blink