Add position sticky main thread scrolling reason [spv2]

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>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 2734 matching lines @@
     return *rareData.filterInfo;
 }
 
 void PaintLayer::removeAncestorOverflowLayer(const PaintLayer* removedLayer)
 {
     // If the current ancestor overflow layer does not match the removed layer
     // the ancestor overflow layer has changed so we can stop searching.
     if (ancestorOverflowLayer() && ancestorOverflowLayer() != removedLayer)
         return;
 
-    if (ancestorOverflowLayer())
+    if (ancestorOverflowLayer()) {
+        // TODO(pdr): When slimming paint v2 is enabled, we will need to
+        // invalidate the scroll paint property subtree for this so main
+        // thread scroll reasons are recomputed.
         ancestorOverflowLayer()->getScrollableArea()->invalidateStickyConstraintsFor(this);
+    }
     updateAncestorOverflowLayer(nullptr);
     PaintLayer* current = m_first;
     while (current) {
         current->removeAncestorOverflowLayer(removedLayer);
         current = current->nextSibling();
     }
 }
 
 void PaintLayer::updateOrRemoveFilterClients()
 {
@@ skipping 235 matching lines @@
 
 void showLayerTree(const blink::LayoutObject* layoutObject)
 {
     if (!layoutObject) {
         fprintf(stderr, "Cannot showLayerTree. Root is (nil)\n");
         return;
     }
     showLayerTree(layoutObject->enclosingLayer());
 }
 #endif