Track size of incremental marking steps in GCTracer.

src/incremental-marking.cc

Index: src/incremental-marking.cc
diff --git a/src/incremental-marking.cc b/src/incremental-marking.cc
index 4150b19f94f13a1a77351e23f5e75e02ba9503aa..0ab9a002452e94aa7846816d14c73e9c4f349b7e 100644
--- a/src/incremental-marking.cc
+++ b/src/incremental-marking.cc
@@ -955,9 +955,9 @@ void IncrementalMarking::Step(intptr_t allocated_bytes,
     }
 
     double end = base::OS::TimeCurrentMillis();
-    double delta = (end - start);
-    heap_->tracer()->AddIncrementalMarkingStep(delta);
-    heap_->AddMarkingTime(delta);
+    double duration = (end - start);
+    heap_->tracer()->AddIncrementalMarkingStep(duration, allocated_bytes);
+    heap_->AddMarkingTime(duration);
   }
 }