WIP: Skeleton for paint offset caching

Source/platform/graphics/paint/DisplayItemList.h

Index: Source/platform/graphics/paint/DisplayItemList.h
diff --git a/Source/platform/graphics/paint/DisplayItemList.h b/Source/platform/graphics/paint/DisplayItemList.h
index ea3ad1b11ace8ecf07f55088e74860d0a826f3c0..46c61a39118ef104f7d919702c53965249d0e741 100644
--- a/Source/platform/graphics/paint/DisplayItemList.h
+++ b/Source/platform/graphics/paint/DisplayItemList.h
@@ -6,6 +6,7 @@
 #define DisplayItemList_h
 
 #include "platform/PlatformExport.h"
+#include "platform/geometry/LayoutPoint.h"
 #include "platform/graphics/ContiguousContainer.h"
 #include "platform/graphics/paint/DisplayItem.h"
 #include "platform/graphics/paint/Transform3DDisplayItem.h"
@@ -46,6 +47,12 @@ public:
     void invalidate(DisplayItemClient);
     void invalidateAll();
 
+    // Record when paint offsets change during paint.
+    void invalidatePaintOffset(DisplayItemClient);
+#if ENABLE(ASSERT)
+    bool paintOffsetWasInvalidated(DisplayItemClient) const;
+#endif
+
     // These methods are called during painting.
     template <typename DisplayItemClass, typename... Args>
     DisplayItemClass& createAndAppend(Args&&... args)
@@ -124,6 +131,7 @@ private:
     friend class DisplayItemListPaintTest;
     friend class DisplayItemListPaintTestForSlimmingPaintV2;
     friend class LayoutObjectDrawingRecorderTest;
+    friend class LayoutObjectDrawingRecorderTestForSlimmingPaintV2;
 
     // Set new item state (scopes, cache skipping, etc) for a new item.
     // TODO(pdr): This only passes a pointer to make the patch easier to review. Change to a reference.
@@ -166,6 +174,12 @@ private:
     mutable HashSet<DisplayItemClient> m_validlyCachedClients;
     mutable bool m_validlyCachedClientsDirty;
 
+#if ENABLE(ASSERT)
+    // Set of clients which had paint offset changes since the last commit. This is used for
+    // ensuring paint offsets are only updated once and are the same in all phases.
+    HashSet<DisplayItemClient> m_clientsWithPaintOffsetInvalidations;
+#endif
+
     // Allow display item construction to be disabled to isolate the costs of construction
     // in performance metrics.
     bool m_constructionDisabled;