Extract code to mark an object during incremental marking

src/heap/incremental-marking.h

Index: src/heap/incremental-marking.h
diff --git a/src/heap/incremental-marking.h b/src/heap/incremental-marking.h
index d6dfe17c7f2472ebb9ce76bdab694e1cb5a00708..c728815b32a28fdce69087ff04e3a4f672d3f872 100644
--- a/src/heap/incremental-marking.h
+++ b/src/heap/incremental-marking.h
@@ -189,6 +189,10 @@ class IncrementalMarking {
 
   bool IsIdleMarkingDelayCounterLimitReached();
 
+  INLINE(static void MarkObject(Heap* heap, Object* object));
+
+  Heap* heap() const { return heap_; }
+
  private:
   int64_t SpaceLeftInOldSpace();