Add tracing for ContentLayerDelegate paintContents and DisplayItemList commit.

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 14000ac5b636cc724c63b4c663a27edf4351b88f..6140393706b7623c5d446e44b6535b85afe53190 100644
--- a/Source/platform/graphics/paint/DisplayItemList.h
+++ b/Source/platform/graphics/paint/DisplayItemList.h
@@ -74,7 +74,8 @@ protected:
     DisplayItemList()
         : m_validlyCachedClientsDirty(false)
         , m_constructionDisabled(false)
-        , m_skippingCacheCount(0) { }
+        , m_skippingCacheCount(0)
+        , m_numCachedItems(0) { }
 
 private:
     friend class DisplayItemListTest;
@@ -121,6 +122,8 @@ private:
 
     int m_skippingCacheCount;
 
+    int m_numCachedItems;
+
     // Scope ids are allocated per client to ensure that the ids are stable for non-invalidated
     // clients between frames, so that we can use the id to match new display items to cached
     // display items.