Move use of DisplayItemList's vector behind an explicit DisplayItems interface.

Source/platform/graphics/paint/TransformDisplayItem.h

Index: Source/platform/graphics/paint/TransformDisplayItem.h
diff --git a/Source/platform/graphics/paint/TransformDisplayItem.h b/Source/platform/graphics/paint/TransformDisplayItem.h
index e0a2c98c03eda5aeefbe83172e1b96de78650d86..ebf36ac25c0e909c6b41265431b972d9160dec79 100644
--- a/Source/platform/graphics/paint/TransformDisplayItem.h
+++ b/Source/platform/graphics/paint/TransformDisplayItem.h
@@ -50,7 +50,7 @@ public:
 
 private:
 #if ENABLE(ASSERT)
-    virtual bool isEndAndPairedWith(const DisplayItem& other) const override final { return other.type() == BeginTransform; }
+    virtual bool isEndAndPairedWith(DisplayItem::Type otherType) const override final { return otherType == BeginTransform; }
 #endif
 };