WIP for display item list backed by ListContainer

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 6140393706b7623c5d446e44b6535b85afe53190..5409b43b3aa8efe09c104033ce9b9ae83d9f6c63 100644
--- a/Source/platform/graphics/paint/DisplayItemList.h
+++ b/Source/platform/graphics/paint/DisplayItemList.h
@@ -16,6 +16,8 @@ namespace blink {
 
 class GraphicsContext;
 
+static const size_t kInitialDisplayItemsCapacity = 32;
+
 class PLATFORM_EXPORT DisplayItemList {
     WTF_MAKE_NONCOPYABLE(DisplayItemList);
     WTF_MAKE_FAST_ALLOCATED(DisplayItemList);
@@ -30,7 +32,15 @@ public:
     void invalidateAll();
 
     // These methods are called during painting.
-    void add(WTF::PassOwnPtr<DisplayItem>);
+    template<typename DisplayItemClass>
+    DisplayItemClass& createAndAppend(const DisplayItemClientWrapper& client, DisplayItem::Type type)
+    {
+        DisplayItemClass& item = m_newDisplayItems.createAndAppend<DisplayItemClass>();
+        item.setClientAndType(client, type);
+        processNewItem(&item);
+        return item;
+    }
+    void add(WTF::PassOwnPtr<DisplayItem>) { } // REMOVE ME
     void beginScope(DisplayItemClient);
     void endScope(DisplayItemClient);
 
@@ -72,7 +82,9 @@ public:
 
 protected:
     DisplayItemList()
-        : m_validlyCachedClientsDirty(false)
+        : m_currentDisplayItems(kInitialDisplayItemsCapacity)
+        , m_newDisplayItems(kInitialDisplayItemsCapacity)
+        , m_validlyCachedClientsDirty(false)
         , m_constructionDisabled(false)
         , m_skippingCacheCount(0)
         , m_numCachedItems(0) { }
@@ -82,6 +94,10 @@ private:
     friend class DisplayItemListPaintTest;
     friend class LayoutObjectDrawingRecorderTest;
 
+    // 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.
+    void processNewItem(DisplayItem*);
+
     void updateValidlyCachedClientsIfNeeded() const;
 
 #ifndef NDEBUG