Blink-side contiguous allocation of display items.

Source/platform/graphics/ListContainer.h

Index: Source/platform/graphics/ListContainer.h
diff --git a/Source/platform/graphics/ListContainer.h b/Source/platform/graphics/ListContainer.h
index 6d5f22ddc4772866346cdad32741666e34cbecd2..9da1ebf225c07b687da75d7e27789f607e3155a3 100644
--- a/Source/platform/graphics/ListContainer.h
+++ b/Source/platform/graphics/ListContainer.h
@@ -9,6 +9,7 @@
 #include "wtf/Forward.h"
 #include "wtf/Noncopyable.h"
 #include "wtf/OwnPtr.h"
+#include "wtf/Utility.h"
 #include "wtf/Vector.h"
 
 #include <iterator>
@@ -234,18 +235,29 @@ public:
 
     BaseElementType* elementAt(size_t index)
     {
-        return *Iterator(iteratorAt(index));
+        return *iteratorAt(index);
     }
+
     const BaseElementType* elementAt(size_t index) const
     {
-        return *ConstIterator(iteratorAt(index));
+        return *iteratorAt(index);
+    }
+
+    Iterator iteratorAt(size_t index)
+    {
+        return Iterator(ListContainerBase::iteratorAt(index));
+    }
+
+    ConstIterator iteratorAt(size_t index) const
+    {
+        return ConstIterator(ListContainerBase::iteratorAt(index));
     }
 
     // Take in derived element type and construct it at location generated by Allocate().
-    template <typename DerivedElementType>
-    DerivedElementType* allocateAndConstruct()
+    template <typename DerivedElementType, typename... Args>
+    DerivedElementType* allocateAndConstruct(Args&&... args)
     {
-        return new (allocate(sizeof(DerivedElementType))) DerivedElementType;
+        return new (allocate(sizeof(DerivedElementType))) DerivedElementType(WTF::forward<Args>(args)...);
     }
 
     // Take in derived element type and copy construct it at location generated by
@@ -273,15 +285,17 @@ public:
     // Appends a new item without copying. The original item will not be
     // destructed and will be replaced with a new DerivedElementType. The
     // DerivedElementType does not have to match the moved type as a full block
-    // of memory will be moved (up to maxSizeForDerivedClass()).
+    // of memory will be moved (up to maxSizeForDerivedClass()). A pointer
+    // to the moved element is returned.
     template <typename DerivedElementType>
-    void appendByMoving(DerivedElementType* item)
+    DerivedElementType* appendByMoving(DerivedElementType* item)
     {
         size_t maxSize = maxSizeForDerivedClass();
         void* newItem = allocate(maxSize);
         memcpy(newItem, static_cast<void*>(item), maxSize);
         // Construct a new element in-place so it can be destructed safely.
         new (item) DerivedElementType;
+        return static_cast<DerivedElementType*>(newItem);
     }
 
     using ListContainerBase::size;
@@ -326,6 +340,7 @@ public:
         friend Iterator ListContainer<BaseElementType>::begin();
         friend Iterator ListContainer<BaseElementType>::end();
         friend BaseElementType* ListContainer<BaseElementType>::elementAt(size_t index);
+        friend Iterator ListContainer<BaseElementType>::iteratorAt(size_t index);
     };
 
     class ConstIterator : public ListContainerBase::ConstIterator {
@@ -364,6 +379,7 @@ public:
         friend ConstIterator ListContainer<BaseElementType>::cbegin() const;
         friend ConstIterator ListContainer<BaseElementType>::cend() const;
         friend const BaseElementType* ListContainer<BaseElementType>::elementAt(size_t index) const;
+        friend ConstIterator ListContainer<BaseElementType>::iteratorAt(size_t index) const;
     };
 
     class ReverseIterator : public ListContainerBase::ReverseIterator {