[heap] Account mark-compact prologue and epilogue in GC tracer.

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index bc501b066e949b933644083f10dc6d49f6a7df85..b2529bd64f4d8091977f6ad7e123c68328be80d1 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -1447,6 +1447,7 @@ void Heap::MarkCompact() {
 
 
 void Heap::MarkCompactEpilogue() {
+  TRACE_GC(tracer(), GCTracer::Scope::MC_EPILOGUE);
   gc_state_ = NOT_IN_GC;
 
   isolate_->counters()->objs_since_last_full()->Set(0);
@@ -1465,6 +1466,7 @@ void Heap::MarkCompactEpilogue() {
 
 
 void Heap::MarkCompactPrologue() {
+  TRACE_GC(tracer(), GCTracer::Scope::MC_PROLOGUE);
   isolate_->context_slot_cache()->Clear();
   isolate_->descriptor_lookup_cache()->Clear();
   RegExpResultsCache::Clear(string_split_cache());