Use an allocation site scratchpad to speed up allocaton site processing during gc.

src/heap-inl.h

Index: src/heap-inl.h
diff --git a/src/heap-inl.h b/src/heap-inl.h
index 3229aeecc3096fa74bc9f022616eb277b4e79d58..1ddaeaf16fd930f802582b9b062dbd3451993576 100644
--- a/src/heap-inl.h
+++ b/src/heap-inl.h
@@ -489,7 +489,15 @@ void Heap::UpdateAllocationSiteFeedback(HeapObject* object) {
         JSObject::cast(object), true);
     if (memento != NULL) {
       ASSERT(memento->IsValid());
-      memento->GetAllocationSite()->IncrementMementoFoundCount();
+      bool add_to_scratchpad =
+          memento->GetAllocationSite()->IncrementMementoFoundCount();
+      Heap* heap = object->GetIsolate()->heap();
+      if (add_to_scratchpad && heap->allocation_sites_scratchpad_length <
+              kAllocationSiteScratchpadSize) {
+        heap->allocation_sites_scratchpad[
+            heap->allocation_sites_scratchpad_length++] =
+                memento->GetAllocationSite();
+      }
     }
   }
 }