GC: Refactor incremental marking steps w/ deadline into a separate call

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 089fe7878c7a2baf6fc279d9e54c30aff2c4d6e8..804a25198ad7d6532545a0abeb0da18caede7beb 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -2291,6 +2291,10 @@ class Heap {
 
   GCIdleTimeHandler::HeapState ComputeHeapState();
 
+  double AdvanceIncrementalMarking(
+      intptr_t step_size_in_bytes, double deadline_in_ms,
+      IncrementalMarking::ForceCompletionAction completion);
+
   bool PerformIdleTimeAction(GCIdleTimeAction action,
                              GCIdleTimeHandler::HeapState heap_state,
                              double deadline_in_ms);