Implemented an averaging object histogram in the vm.

runtime/vm/heap.cc

Index: runtime/vm/heap.cc
===================================================================
--- runtime/vm/heap.cc	(revision 23906)
+++ runtime/vm/heap.cc	(working copy)
@@ -7,6 +7,7 @@
 #include "platform/assert.h"
 #include "platform/utils.h"
 #include "vm/flags.h"
+#include "vm/heap_histogram.h"
 #include "vm/heap_profiler.h"
 #include "vm/isolate.h"
 #include "vm/object.h"
@@ -162,6 +163,7 @@
       old_space_->MarkSweep(invoke_api_callbacks);
       RecordAfterGC();
       PrintStats();
+      UpdateObjectHistogram();
       break;
     }
     default:
@@ -170,6 +172,13 @@
 }
 
 
+void Heap::UpdateObjectHistogram() {
+  Isolate* isolate = Isolate::Current();
+  if (isolate->object_histogram() == NULL) return;
+  isolate->object_histogram()->Collect();
+}
+
+
 void Heap::CollectGarbage(Space space) {
   ApiCallbacks api_callbacks;
   if (space == kOld) {
@@ -190,6 +199,7 @@
   old_space_->MarkSweep(kInvokeApiCallbacks);
   RecordAfterGC();
   PrintStats();
+  UpdateObjectHistogram();
 }