Unify markbits for old and new spaces.

src/assembler.cc

Index: src/assembler.cc
diff --git a/src/assembler.cc b/src/assembler.cc
index e8ca5d811a6b6b83a7223bf80d5ed301ca344854..0c5a456a0a20fd48bb0e4c309d11928bc386db72 100644
--- a/src/assembler.cc
+++ b/src/assembler.cc
@@ -843,11 +843,6 @@ ExternalReference ExternalReference::new_space_start(Isolate* isolate) {
 }
 
 
-ExternalReference ExternalReference::new_space_mark_bits(Isolate* isolate) {
-  return ExternalReference(isolate->heap()->marking()->new_space_bitmap());
-}
-
-
 ExternalReference ExternalReference::store_buffer_top(Isolate* isolate) {
   return ExternalReference(isolate->heap()->store_buffer()->TopAddress());
 }