Add experimental support for tracing the state of the VM heap to a file

src/incremental-marking.cc

Index: src/incremental-marking.cc
===================================================================
--- src/incremental-marking.cc	(revision 9457)
+++ src/incremental-marking.cc	(working copy)
@@ -686,11 +686,13 @@
   }
 
   if (state_ == SWEEPING) {
+    heap_->VisualizerTimeStamp(HeapVisualizer::kSweeping);
     if (heap_->old_pointer_space()->AdvanceSweeper(bytes_to_process) &&
         heap_->old_data_space()->AdvanceSweeper(bytes_to_process)) {
       StartMarking();
     }
   } else if (state_ == MARKING) {
+    heap_->VisualizerTimeStamp(HeapVisualizer::kMarking);
     Map* filler_map = heap_->one_pointer_filler_map();
     Map* global_context_map = heap_->global_context_map();
     IncrementalMarkingMarkingVisitor marking_visitor(heap_, this);
@@ -778,6 +780,7 @@
     steps_took_ += delta;
     steps_took_since_last_gc_ += delta;
   }
+  heap_->VisualizerTimeStamp(HeapVisualizer::kRunning);
 }