Only promote fixed position elements on low dpi without transform or opacity.

third_party/WebKit/Source/core/paint/PaintLayer.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Intel Corporation. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
@@ skipping 541 matching lines @@
       const LayoutObject&,
       const LayoutBoxModelObject& paintInvalidationContainer,
       LayoutRect&);
 
   bool paintsWithTransparency(GlobalPaintFlags globalPaintFlags) const {
     return isTransparent() &&
            ((globalPaintFlags & GlobalPaintFlattenCompositingLayers) ||
             compositingState() != PaintsIntoOwnBacking);
   }
 
+  // Returns true if the element or any ancestor is transformed.
+  bool compositesWithTransform() const;
+
+  // Returns true if the element or any ancestor has non 1 opacity.
+  bool compositesWithOpacity() const;
+
   bool paintsWithTransform(GlobalPaintFlags) const;
 
   // Returns true if background phase is painted opaque in the given rect.
   // The query rect is given in local coordinates.
   bool backgroundIsKnownToBeOpaqueInRect(const LayoutRect&) const;
 
   bool containsDirtyOverlayScrollbars() const {
     return m_containsDirtyOverlayScrollbars;
   }
   void setContainsDirtyOverlayScrollbars(bool dirtyScrollbars) {
@@ skipping 665 matching lines @@
 
 }  // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 CORE_EXPORT void showLayerTree(const blink::PaintLayer*);
 CORE_EXPORT void showLayerTree(const blink::LayoutObject*);
 #endif
 
 #endif  // Layer_h