[heap] Introduce enum of garbage collection reasons.

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 2d437b1b7d249910a313c3de207481b2a7b22430..ae0776a50cdde25ce0427e97124a3db80a3254db 100644
--- a/src/heap/memory-reducer.cc
+++ b/src/heap/memory-reducer.cc
@@ -73,7 +73,8 @@ void MemoryReducer::NotifyTimer(const Event& event) {
       PrintIsolate(heap()->isolate(), "Memory reducer: started GC #%d\n",
                    state_.started_gcs);
     }
-    heap()->StartIdleIncrementalMarking();
+    heap()->StartIdleIncrementalMarking(
+        GarbageCollectionReason::kMemoryReducer);
   } else if (state_.action == kWait) {
     if (!heap()->incremental_marking()->IsStopped() &&
         heap()->ShouldOptimizeForMemoryUsage()) {
@@ -87,7 +88,7 @@ void MemoryReducer::NotifyTimer(const Event& event) {
           deadline, IncrementalMarking::NO_GC_VIA_STACK_GUARD,
           IncrementalMarking::FORCE_COMPLETION);
       heap()->FinalizeIncrementalMarkingIfComplete(
-          "Memory reducer: finalize incremental marking");
+          GarbageCollectionReason::kFinalizeMarkingViaTask);
     }
     // Re-schedule the timer.
     ScheduleTimer(event.time_ms, state_.next_gc_start_ms - event.time_ms);