[heap] Make the Marking class all static.

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 6666b2c8238189ccfe269a8616c660550a69fe6a..2ac562fb22829d1ddc869a095963f4c22389adfd 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -1426,8 +1426,6 @@ class Heap {
 
   StoreBuffer* store_buffer() { return &store_buffer_; }
 
-  Marking* marking() { return &marking_; }
-
   IncrementalMarking* incremental_marking() { return &incremental_marking_; }
 
   ExternalStringTable* external_string_table() {
@@ -2300,8 +2298,6 @@ class Heap {
 
   StoreBuffer store_buffer_;
 
-  Marking marking_;
-
   IncrementalMarking incremental_marking_;
 
   GCIdleTimeHandler gc_idle_time_handler_;