Only store previous clip rects for PaintLayers that support subsequences.

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

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. 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 2730 matching lines @@
            ((global_paint_flags & kGlobalPaintFlattenCompositingLayers) ||
             GetCompositingState() != kPaintsIntoOwnBacking);
   }
 
   return (Transform() ||
           GetLayoutObject().Style()->GetPosition() == EPosition::kFixed) &&
          ((global_paint_flags & kGlobalPaintFlattenCompositingLayers) ||
           GetCompositingState() != kPaintsIntoOwnBacking);
 }
 
+bool PaintLayer::SupportsSubsequenceCaching() const {
+  // SVG paints atomically.
+  if (GetLayoutObject().IsSVGRoot())
+    return true;
+
+  // Create subsequence for only stacking contexts whose painting are atomic.
+  if (!StackingNode()->IsStackingContext())
+    return false;
+
+  // The layer doesn't have children. Subsequence caching is not worth it,
+  // because normally the actual painting will be cheap.
+  // SVG is also painted atomically.
+  if (!PaintLayerStackingNodeIterator(*StackingNode(), kAllChildren).Next())
+    return false;
+
+  return true;
+}
+
 ScrollingCoordinator* PaintLayer::GetScrollingCoordinator() {
   Page* page = GetLayoutObject().GetFrame()->GetPage();
   return (!page) ? nullptr : page->GetScrollingCoordinator();
 }
 
 bool PaintLayer::CompositesWithTransform() const {
   return TransformAncestor() || Transform();
 }
 
 bool PaintLayer::CompositesWithOpacity() const {
@@ skipping 522 matching lines @@
 }
 
 void showLayerTree(const blink::LayoutObject* layoutObject) {
   if (!layoutObject) {
     LOG(INFO) << "Cannot showLayerTree. Root is (nil)";
     return;
   }
   showLayerTree(layoutObject->EnclosingLayer());
 }
 #endif