Clean-up and repair cumulative marking and sweeping time stats.

src/gc-tracer.cc

Index: src/gc-tracer.cc
diff --git a/src/gc-tracer.cc b/src/gc-tracer.cc
index 74a2aa873f1cb635ad6a2401c01cc4640db7f1c0..56b359df37e01108fd58c0f1e530520e5da38bea 100644
--- a/src/gc-tracer.cc
+++ b/src/gc-tracer.cc
@@ -75,7 +75,9 @@ GCTracer::GCTracer(Heap* heap)
       cumulative_incremental_marking_steps_(0),
       cumulative_incremental_marking_bytes_(0),
       cumulative_incremental_marking_duration_(0.0),
-      longest_incremental_marking_step_(0.0) {
+      longest_incremental_marking_step_(0.0),
+      cumulative_marking_duration_(0.0),
+      cumulative_sweeping_duration_(0.0) {
   current_ = Event(Event::START, NULL, NULL);
   current_.end_time = base::OS::TimeCurrentMillis();
   previous_ = previous_mark_compactor_event_ = current_;
@@ -174,6 +176,7 @@ void GCTracer::AddIncrementalMarkingStep(double duration, intptr_t bytes) {
   cumulative_incremental_marking_duration_ += duration;
   longest_incremental_marking_step_ =
       Max(longest_incremental_marking_step_, duration);
+  cumulative_marking_duration_ += duration;
 }