WIP: Skeleton for paint offset caching

Source/core/layout/LayoutObject.h

Index: Source/core/layout/LayoutObject.h
diff --git a/Source/core/layout/LayoutObject.h b/Source/core/layout/LayoutObject.h
index 06e8d0270fba1af8f836169364b4e2f30ebf0d3b..059f11ca016054bb40673ea5d065996180ac2b58 100644
--- a/Source/core/layout/LayoutObject.h
+++ b/Source/core/layout/LayoutObject.h
@@ -1009,6 +1009,10 @@ public:
     const LayoutPoint& previousPositionFromPaintInvalidationBacking() const { return m_previousPositionFromPaintInvalidationBacking; }
     void setPreviousPositionFromPaintInvalidationBacking(const LayoutPoint& positionFromPaintInvalidationBacking) { m_previousPositionFromPaintInvalidationBacking = positionFromPaintInvalidationBacking; }
 
+    // Cache of the previous paint offset for slimming paint v2.
+    const LayoutPoint& cachedPaintOffset() const;
+    void updateCachedPaintOffset(const LayoutPoint&) const;
+
     PaintInvalidationReason fullPaintInvalidationReason() const { return m_bitfields.fullPaintInvalidationReason(); }
     bool shouldDoFullPaintInvalidation() const { return m_bitfields.fullPaintInvalidationReason() != PaintInvalidationNone; }
     void setShouldDoFullPaintInvalidation(PaintInvalidationReason = PaintInvalidationFull);
@@ -1427,6 +1431,11 @@ private:
     // It is used to detect layoutObject shifts that forces a full invalidation.
     // This point does *not* account for composited scrolling. See adjustInvalidationRectForCompositedScrolling().
     LayoutPoint m_previousPositionFromPaintInvalidationBacking;
+
+    // Previous paint offset for slimming paint v2.
+    // TODO(pdr): This should be merged with m_previousPositionFromPaintInvalidationBacking (https://codereview.chromium.org/1315213002).
+    // DO NOT COMMIT until https://codereview.chromium.org/1315213002 lands.
+    mutable LayoutPoint m_cachedPaintOffset;
 };
 
 // FIXME: remove this once the layout object lifecycle ASSERTS are no longer hit.