[heap] Implement concurrent marking boilerplate.

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index c504237baf9351cc2f156c257b04e45601a09ae2..b50950e477ea93702649aec0d1b2f8caa4b503d8 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -318,6 +318,7 @@ using v8::MemoryPressureLevel;
 // Forward declarations.
 class AllocationObserver;
 class ArrayBufferTracker;
+class ConcurrentMarking;
 class GCIdleTimeAction;
 class GCIdleTimeHandler;
 class GCIdleTimeHeapState;
@@ -2290,6 +2291,7 @@ class Heap {
   StoreBuffer* store_buffer_;
 
   IncrementalMarking* incremental_marking_;
+  ConcurrentMarking* concurrent_marking_;
 
   GCIdleTimeHandler* gc_idle_time_handler_;
 
@@ -2373,6 +2375,7 @@ class Heap {
 
   // Classes in "heap" can be friends.
   friend class AlwaysAllocateScope;
+  friend class ConcurrentMarking;
   friend class GCCallbacksScope;
   friend class GCTracer;
   friend class HeapIterator;