v8: make GCTracer persistent.

src/global-handles.cc

Index: src/global-handles.cc
diff --git a/src/global-handles.cc b/src/global-handles.cc
index a5ae2d5626f3701f8c953676ebc2a3e1932736fb..cf3d760a62d368d8b2700c31db5597663e076e6d 100644
--- a/src/global-handles.cc
+++ b/src/global-handles.cc
@@ -612,7 +612,7 @@ bool GlobalHandles::IterateObjectGroups(ObjectVisitor* v,
 
 
 int GlobalHandles::PostGarbageCollectionProcessing(
-    GarbageCollector collector, GCTracer* tracer) {
+    GarbageCollector collector) {
   // Process weak global handle callbacks. This must be done after the
   // GC is completely done, because the callbacks may invoke arbitrary
   // API functions.
@@ -675,14 +675,14 @@ int GlobalHandles::PostGarbageCollectionProcessing(
     if (node->IsRetainer()) {
       if (isolate_->heap()->InNewSpace(node->object())) {
         new_space_nodes_[last++] = node;
-        tracer->increment_nodes_copied_in_new_space();
+        isolate_->heap()->tracer()->increment_nodes_copied_in_new_space();
       } else {
         node->set_in_new_space_list(false);
-        tracer->increment_nodes_promoted();
+        isolate_->heap()->tracer()->increment_nodes_promoted();
       }
     } else {
       node->set_in_new_space_list(false);
-      tracer->increment_nodes_died_in_new_space();
+      isolate_->heap()->tracer()->increment_nodes_died_in_new_space();
     }
   }
   new_space_nodes_.Rewind(last);