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

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..4feab87ac61056353652524eeb5025cb9720d622 100644
--- a/Source/platform/graphics/paint/DisplayItem.h
+++ b/Source/platform/graphics/paint/DisplayItem.h
@@ -18,6 +18,7 @@
 
 namespace blink {
 
+class DisplayItems;
 class GraphicsContext;
 class WebDisplayItemList;
 
@@ -233,6 +234,7 @@ public:
     bool skippedCache() const { return m_skippedCache; }
 
     virtual void appendToWebDisplayItemList(WebDisplayItemList*) const { }
+    virtual void appendByMoving(DisplayItems&) = 0;
 
     // See comments of enum Type for usage of the following macros.
 #define DEFINE_CATEGORY_METHODS(Category) \
@@ -333,6 +335,10 @@ private:
 #ifndef NDEBUG
     WTF::String m_clientDebugString;
 #endif
+
+    // TODO(jbroman): Copying this data in appendByMoving would be better than
+    // friendship, but this is just hacked up.
+    friend class DisplayItems;
 };
 
 class PLATFORM_EXPORT PairedBeginDisplayItem : public DisplayItem {
@@ -355,6 +361,27 @@ private:
     virtual bool isEnd() const override final { return true; }
 };
 
+// Allows a DisplayItemClientWrapper to be constructed from a DisplayItem, in
+// order to facilitate copying it.
+// TODO(jbroman): This is a bad class name.
+class DisplayItemClientWrapperHelper {
+    DISALLOW_ALLOCATION();
+public:
+    DisplayItemClientWrapperHelper(const DisplayItem& wrapped) : m_wrapped(wrapped) { }
+    DisplayItemClient displayItemClient() const { return m_wrapped.client(); }
+    String debugName() const
+    {
+#ifndef NDEBUG
+        return m_wrapped.clientDebugString();
+#else
+        return String();
+#endif
+    }
+private:
+    const DisplayItem& m_wrapped;
+};
+
+
 } // namespace blink
 
 #endif // DisplayItem_h