WIP for display item list backed by ListContainer

Source/platform/graphics/paint/DisplayItem.h

Index: Source/platform/graphics/paint/DisplayItem.h
diff --git a/Source/platform/graphics/paint/DisplayItem.h b/Source/platform/graphics/paint/DisplayItem.h
index ed8985384fd6aee80bffa12ab56447a5271950e0..dce94f8c88e84c79c4ded2163dec47bfe154557c 100644
--- a/Source/platform/graphics/paint/DisplayItem.h
+++ b/Source/platform/graphics/paint/DisplayItem.h
@@ -23,6 +23,17 @@ class WebDisplayItemList;
 
 class PLATFORM_EXPORT DisplayItem {
 public:
+    DisplayItem()
+        : m_client(nullptr)
+        , m_scopeContainer(nullptr)
+        , m_scopeId(0)
+        , m_type(UninitializedType)
+        , m_skippedCache(false)
+#ifndef NDEBUG
+        , m_clientDebugString("invalid")
+#endif
+    { }
+
     enum {
         // Must be kept in sync with core/layout/PaintPhase.h.
         PaintPhaseMax = 12,
@@ -173,7 +184,8 @@ public:
         EndSubtreePaintPhaseLast = EndSubtreePaintPhaseFirst + PaintPhaseMax,
         EndSubtreeLast = EndSubtreePaintPhaseLast,
 
-        TypeLast = EndSubtreeLast
+        UninitializedType,
+        TypeLast = UninitializedType
     };
 
     struct Id {
@@ -311,23 +323,29 @@ public:
 #endif
 
 protected:
-    DisplayItem(const DisplayItemClientWrapper& client, Type type)
-        : m_client(client.displayItemClient())
-        , m_scopeContainer(nullptr)
-        , m_scopeId(0)
-        , m_type(type)
-        , m_skippedCache(false)
+    // TODO(pdr): friend just the function DisplayItemList::createAndAppend(...).
+    friend class DisplayItemList;
+    void setClientAndType(const DisplayItemClientWrapper& clientWrapper, Type type)
+    {
+        // Clients should never change once set.
+        ASSERT(m_client == nullptr);
+        m_client = clientWrapper.displayItemClient();
+
+        // Types should never change once set.
+        ASSERT(m_type = UninitializedType);
+        m_type = type;
+
 #ifndef NDEBUG
-        , m_clientDebugString(client.debugName())
+        m_clientDebugString = clientWrapper.debugName();
 #endif
-    { }
+    }
 
 private:
-    const DisplayItemClient m_client;
+    DisplayItemClient m_client;
     DisplayItemClient m_scopeContainer;
     int m_scopeId;
     static_assert(TypeLast < (1 << 16), "DisplayItem::Type should fit in 16 bits");
-    const Type m_type : 16;
+    Type m_type : 16;
     unsigned m_skippedCache : 1;
 
 #ifndef NDEBUG
@@ -337,7 +355,7 @@ private:
 
 class PLATFORM_EXPORT PairedBeginDisplayItem : public DisplayItem {
 protected:
-    PairedBeginDisplayItem(const DisplayItemClientWrapper& client, Type type) : DisplayItem(client, type) { }
+    PairedBeginDisplayItem() { }
 
 private:
     virtual bool isBegin() const override final { return true; }
@@ -345,7 +363,7 @@ private:
 
 class PLATFORM_EXPORT PairedEndDisplayItem : public DisplayItem {
 protected:
-    PairedEndDisplayItem(const DisplayItemClientWrapper& client, Type type) : DisplayItem(client, type) { }
+    PairedEndDisplayItem() { }
 
 #if ENABLE(ASSERT)
     virtual bool isEndAndPairedWith(DisplayItem::Type otherType) const override = 0;