[heap] Tracer: Fix accounting for external epilogue

R=hpayer@chromium.org
LOG=N
BUG=

Review URL: https://codereview.chromium.org/1836283007

Cr-Commit-Position: refs/heads/master@{#35169}

src/heap/heap.cc

@@ -841,7 +841,7 @@ void Heap::FinalizeIncrementalMarking(const char* gc_reason) {
     if (scope.CheckReenter()) {
       AllowHeapAllocation allow_allocation;
       GCTracer::Scope scope(tracer(),
-                            GCTracer::Scope::MC_INCREMENTAL_EXTERNAL_PROLOGUE);
+                            GCTracer::Scope::MC_INCREMENTAL_EXTERNAL_EPILOGUE);
       VMState<EXTERNAL> state(isolate_);
       HandleScope handle_scope(isolate_);
       CallGCEpilogueCallbacks(kGCTypeIncrementalMarking, kNoGCCallbackFlags);