Slimming Paint phase 2 compositing algorithm plumbing

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 4c3ef97d74167bbd008d0a99cfa7af4a2bcd386f..a8a8306680b16d9fc65cc0bf97f31c825a83af12 100644
--- a/Source/platform/graphics/paint/DisplayItemList.h
+++ b/Source/platform/graphics/paint/DisplayItemList.h
@@ -27,6 +27,10 @@ static const size_t kDisplayItemAlignment = WTF_ALIGN_OF(BeginTransform3DDisplay
 static const size_t kInitialDisplayItemsCapacity = 64;
 static const size_t kMaximumDisplayItemSize = sizeof(BeginTransform3DDisplayItem);
 
+// Map from SimpleLayer.startPoint to the DrawingDisplayItems within its range
+// which were invalidated on this frame and do not change SimpleLayers.
+using DisplayListDiff = HashMap<DisplayItemClient, DisplayItem*>;
+
 using DisplayItems = ContiguousContainer<DisplayItem, kDisplayItemAlignment>;
 
 class PLATFORM_EXPORT DisplayItemList {
@@ -70,8 +74,9 @@ public:
     void endSkippingCache() { ASSERT(m_skippingCacheCount > 0); --m_skippingCacheCount; }
     bool skippingCache() const { return m_skippingCacheCount; }
 
-    // Must be called when a painting is finished.
-    void commitNewDisplayItems();
+    // Must be called when a painting is finished. If passed, a DisplayListDiff
+    // is initialized and created.
+    void commitNewDisplayItems(DisplayListDiff* = 0);
 
     // Returns the approximate memory usage, excluding memory likely to be
     // shared with the embedder after copying to WebDisplayItemList.