Fix accidental performance regression.

Accidentally changed mod union table in another CL, slowed down
partial GC by a bit.

Change-Id: I8aba855a6b475da2969187ecd73f05c0667e470d
diff --git a/runtime/gc/heap.cc b/runtime/gc/heap.cc
index ece9920..3cc60d9 100644
--- a/runtime/gc/heap.cc
+++ b/runtime/gc/heap.cc
@@ -186,7 +186,7 @@
   card_table_.reset(accounting::CardTable::Create(heap_begin, heap_capacity));
   CHECK(card_table_.get() != NULL) << "Failed to create card table";
 
-  image_mod_union_table_.reset(new accounting::ModUnionTableCardCache(this));
+  image_mod_union_table_.reset(new accounting::ModUnionTableToZygoteAllocspace(this));
   CHECK(image_mod_union_table_.get() != NULL) << "Failed to create image mod-union table";
 
   zygote_mod_union_table_.reset(new accounting::ModUnionTableCardCache(this));