Implement a paint offset cache for slimming paint v2

Source/core/layout/LayoutObject.cpp

Index: Source/core/layout/LayoutObject.cpp
diff --git a/Source/core/layout/LayoutObject.cpp b/Source/core/layout/LayoutObject.cpp
index 6ddea1c970ff66b203ede634690f88b51eb529ca..53278ec708b66359f5084139d2a5b7478f3a6675 100644
--- a/Source/core/layout/LayoutObject.cpp
+++ b/Source/core/layout/LayoutObject.cpp
@@ -138,6 +138,7 @@ struct SameSizeAsLayoutObject {
     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");
@@ -3188,6 +3189,18 @@ void LayoutObject::clearPaintInvalidationState(const PaintInvalidationState& pai
     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()