[heap] Remove ability to turn off code flushing on demand.

src/heap/objects-visiting-inl.h

Index: src/heap/objects-visiting-inl.h
diff --git a/src/heap/objects-visiting-inl.h b/src/heap/objects-visiting-inl.h
index 51f7c7ad8147b7dabeff9b8b5c3eac3ed7048dc8..2d1bb839b49382513b0c2b85ed9f005228611b83 100644
--- a/src/heap/objects-visiting-inl.h
+++ b/src/heap/objects-visiting-inl.h
@@ -679,6 +679,11 @@ bool StaticMarkingVisitor<StaticVisitor>::IsFlushable(
     return false;
   }
 
+  // Maintain debug break slots in the code.
+  if (shared_info->HasDebugCode()) {
+    return false;
+  }
+
   // If this is a function initialized with %SetCode then the one-to-one
   // relation between SharedFunctionInfo and Code is broken.
   if (shared_info->dont_flush()) {