Add a flag for memory reducer.

src/heap/memory-reducer.cc

Index: src/heap/memory-reducer.cc
diff --git a/src/heap/memory-reducer.cc b/src/heap/memory-reducer.cc
index 24663dbf618eef6fdbcd6ce92924e4352e98a956..278e8c670b3403da83a7f1355a43cf03b6cb83bf 100644
--- a/src/heap/memory-reducer.cc
+++ b/src/heap/memory-reducer.cc
@@ -150,7 +150,7 @@ bool MemoryReducer::WatchdogGC(const State& state, const Event& event) {
 // For specification of this function see the comment for MemoryReducer class.
 MemoryReducer::State MemoryReducer::Step(const State& state,
                                          const Event& event) {
-  if (!FLAG_incremental_marking) {
+  if (!FLAG_incremental_marking || !FLAG_memory_reducer) {
     return State(kDone, 0, 0, state.last_gc_time_ms);
   }
   switch (state.action) {