[Slimming Paint] Blink-side contiguous allocation of display items.

Source/platform/graphics/paint/DisplayItemListTest.cpp

Index: Source/platform/graphics/paint/DisplayItemListTest.cpp
diff --git a/Source/platform/graphics/paint/DisplayItemListTest.cpp b/Source/platform/graphics/paint/DisplayItemListTest.cpp
index 7156ed6c31e9596a68a20b431604b61dff370ffd..7c5ce34b129e784b7addb31b6328f7df167bed5a 100644
--- a/Source/platform/graphics/paint/DisplayItemListTest.cpp
+++ b/Source/platform/graphics/paint/DisplayItemListTest.cpp
@@ -8,6 +8,7 @@
 #include "platform/RuntimeEnabledFeatures.h"
 #include "platform/graphics/GraphicsContext.h"
 #include "platform/graphics/paint/ClipRecorder.h"
+#include "platform/graphics/paint/DisplayItems.h"
 #include "platform/graphics/paint/DrawingDisplayItem.h"
 #include "platform/graphics/paint/DrawingRecorder.h"
 #include <gtest/gtest.h>
@@ -60,6 +61,12 @@ public:
 
     virtual void replay(GraphicsContext&) override final { ASSERT_NOT_REACHED(); }
     virtual void appendToWebDisplayItemList(WebDisplayItemList*) const override final { ASSERT_NOT_REACHED(); }
+
+    void appendByMoving(DisplayItems& destination) override
+    {
+        destination.emplaceBack<TestDisplayItem>(
+            DisplayItemClientWrapperHelper(*this), type());
+    }
 };
 
 #ifndef NDEBUG