[heap] Refactor incremental marking step.

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 1a567e08be00f66a09bd66bd8af92c1296878c1a..fad146c36fde42a5a93e3ea1741e3db39efefd7f 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -1159,6 +1159,11 @@ class Heap {
                                    GCCallbackFlags::kNoGCCallbackFlags,
                                const char* reason = nullptr);
 
+  void StartIncrementalMarkingIfNeeded(int gc_flags = kNoGCFlags,
+                                       const GCCallbackFlags gc_callback_flags =
+                                           GCCallbackFlags::kNoGCCallbackFlags,
+                                       const char* reason = nullptr);
+
   void FinalizeIncrementalMarkingIfComplete(const char* comment);
 
   bool TryFinalizeIdleIncrementalMarking(double idle_time_in_ms);