Slimming Paint phase 2 compositing algorithm plumbing & skeleton display list API.

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 133e2f8e0b18840e0b0b2a9cdfefe8b825098e20..d95623b61a512dcc202928447d9a49cfc8c901c8 100644
--- a/Source/platform/graphics/paint/DisplayItemList.h
+++ b/Source/platform/graphics/paint/DisplayItemList.h
@@ -23,6 +23,10 @@ using DisplayItems = ListContainer<DisplayItem>;
 static const size_t kInitialDisplayItemsCapacity = 64;
 static const size_t kMaximumDisplayItemSize = sizeof(BeginTransform3DDisplayItem);
 
+class DisplayListDiff {
+    Vector<int> diffOffsets;
+};
+
 class PLATFORM_EXPORT DisplayItemList {
     WTF_MAKE_NONCOPYABLE(DisplayItemList);
     WTF_MAKE_FAST_ALLOCATED(DisplayItemList);
@@ -60,8 +64,9 @@ 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, the diff is initialized
+    // to the difference of the new display list from the last time commitNewDisplayItems() was called.
+    void commitNewDisplayItems(DisplayListDiff* = 0);
 
     // Returns the approximate memory usage, excluding memory likely to be
     // shared with the embedder after copying to WebDisplayItemList.