Implement a paint offset cache for slimming paint v2

Source/core/layout/LayoutObject.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  *           (C) 2004 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Google Inc. All rights reserved.
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 120 matching lines @@
 struct SameSizeAsLayoutObject {
     virtual ~SameSizeAsLayoutObject() { } // Allocate vtable pointer.
     void* pointers[5];
 #if ENABLE(ASSERT)
     unsigned m_debugBitfields : 2;
 #endif
     unsigned m_bitfields;
     unsigned m_bitfields2;
     LayoutRect rect; // Stores the previous paint invalidation rect.
     LayoutPoint position; // Stores the previous position from the paint invalidation container.
+    LayoutPoint paintOffset; // DO NOT COMMIT until https://codereview.chromium.org/1315213002 lands and we can remove this.
 };
 
 static_assert(sizeof(LayoutObject) == sizeof(SameSizeAsLayoutObject), "LayoutObject should stay small");
 
 bool LayoutObject::s_affectsParentBlock = false;
 
 typedef HashMap<const LayoutObject*, LayoutRect> SelectionPaintInvalidationMap;
 static SelectionPaintInvalidationMap* selectionPaintInvalidationMap = nullptr;
 
 void* LayoutObject::operator new(size_t sz)
@@ skipping 3030 matching lines @@
     // booleans that are cleared below.
     ASSERT(paintInvalidationState.ancestorHadPaintInvalidationForLocationChange() || paintInvalidationStateIsDirty());
     clearShouldDoFullPaintInvalidation();
     m_bitfields.setChildShouldCheckForPaintInvalidation(false);
     m_bitfields.setNeededLayoutBecauseOfChildren(false);
     m_bitfields.setShouldInvalidateOverflowForPaint(false);
     m_bitfields.setMayNeedPaintInvalidation(false);
     m_bitfields.setShouldInvalidateSelection(false);
 }
 
+const LayoutPoint& LayoutObject::cachedPaintOffset() const
+{
+    ASSERT(RuntimeEnabledFeatures::slimmingPaintV2Enabled());
+    return m_cachedPaintOffset;
+}
+
+void LayoutObject::updateCachedPaintOffset(const LayoutPoint& paintOffset) const
+{
+    ASSERT(RuntimeEnabledFeatures::slimmingPaintV2Enabled());
+    m_cachedPaintOffset = paintOffset;
+}
+
 bool LayoutObject::isAllowedToModifyLayoutTreeStructure(Document& document)
 {
     return DeprecatedDisableModifyLayoutTreeStructureAsserts::canModifyLayoutTreeStateInAnyState()
         || document.lifecycle().stateAllowsLayoutTreeMutations();
 }
 
 DeprecatedDisableModifyLayoutTreeStructureAsserts::DeprecatedDisableModifyLayoutTreeStructureAsserts()
     : m_disabler(gModifyLayoutTreeStructureAnyState, true)
 {
 }
@@ skipping 183 matching lines @@
         const blink::LayoutObject* root = object1;
         while (root->parent())
             root = root->parent();
         root->showLayoutTreeAndMark(object1, "*", object2, "-", 0);
     } else {
         fprintf(stderr, "Cannot showLayoutTree. Root is (nil)\n");
     }
 }
 
 #endif