[heap] Make the current GCCallbackFlags are part of {Heap}.

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 fcada78f0b225f9cff609ea2134e45ec2eb49104..18c8c0dbb20011e3d427da1e827cfdb6089e397c 100644
--- a/src/heap/incremental-marking.h
+++ b/src/heap/incremental-marking.h
@@ -81,9 +81,7 @@ class IncrementalMarking {
 
   bool WasActivated();
 
-  void Start(int flags,
-             const GCCallbackFlags gc_callback_flags = kNoGCCallbackFlags,
-             const char* reason = nullptr);
+  void Start(const char* reason = nullptr);
 
   void MarkObjectGroups();
 
@@ -199,8 +197,6 @@ class IncrementalMarking {
 
   Heap* heap() const { return heap_; }
 
-  GCCallbackFlags CallbackFlags() const { return gc_callback_flags_; }
-
  private:
   int64_t SpaceLeftInOldSpace();
 
@@ -259,8 +255,6 @@ class IncrementalMarking {
 
   GCRequestType request_type_;
 
-  GCCallbackFlags gc_callback_flags_;
-
   DISALLOW_IMPLICIT_CONSTRUCTORS(IncrementalMarking);
 };
 }