Always trace garbage collection events in GCTracer.

src/gc-tracer.cc

Index: src/gc-tracer.cc
diff --git a/src/gc-tracer.cc b/src/gc-tracer.cc
index 3b381b3ad9c0c4b1843311edbc57122a1a526da0..6119867692599d3e0340e734c80fbbfda7a126ab 100644
--- a/src/gc-tracer.cc
+++ b/src/gc-tracer.cc
@@ -78,8 +78,6 @@ GCTracer::GCTracer(Heap* heap)
 
 void GCTracer::Start(GarbageCollector collector, const char* gc_reason,
                      const char* collector_reason) {
-  if (!FLAG_trace_gc && !FLAG_print_cumulative_gc_stat) return;
-
   previous_ = current_;
   if (current_.type == Event::MARK_COMPACTOR)
     previous_mark_compactor_event_ = current_;
@@ -106,8 +104,6 @@ void GCTracer::Start(GarbageCollector collector, const char* gc_reason,
 
 
 void GCTracer::Stop() {
-  if (!FLAG_trace_gc && !FLAG_print_cumulative_gc_stat) return;
-
   current_.end_time = base::OS::TimeCurrentMillis();
   current_.end_object_size = heap_->SizeOfObjects();
   current_.end_memory_size = heap_->isolate()->memory_allocator()->Size();
@@ -122,6 +118,10 @@ void GCTracer::Stop() {
   if (current_.type == Event::MARK_COMPACTOR)
     longest_incremental_marking_step_ = 0.0;
 
+  // TODO(ernstm): move the code below out of GCTracer.
+
+  if (!FLAG_trace_gc && !FLAG_print_cumulative_gc_stat) return;
+
   double duration = current_.end_time - current_.start_time;
   double spent_in_mutator = Max(current_.start_time - previous_.end_time, 0.0);