Expose last seen heap object id via v8 public api.

src/heap-profiler.h

Index: src/heap-profiler.h
diff --git a/src/heap-profiler.h b/src/heap-profiler.h
index 96b042d3cb49903acd91fa9e812cfac2be7ecbab..4a811573ac1764742432d4e87852682e802d5291 100644
--- a/src/heap-profiler.h
+++ b/src/heap-profiler.h
@@ -58,7 +58,7 @@ class HeapProfiler {
 
   static void StartHeapObjectsTracking();
   static void StopHeapObjectsTracking();
-  static void PushHeapObjectsStats(OutputStream* stream);
+  static SnapshotObjectId PushHeapObjectsStats(OutputStream* stream);
   static int GetSnapshotsCount();
   static HeapSnapshot* GetSnapshot(int index);
   static HeapSnapshot* FindSnapshot(unsigned uid);
@@ -89,7 +89,7 @@ class HeapProfiler {
 
   void StartHeapObjectsTrackingImpl();
   void StopHeapObjectsTrackingImpl();
-  void PushHeapObjectsStatsImpl(OutputStream* stream);
+  SnapshotObjectId PushHeapObjectsStatsImpl(OutputStream* stream);
 
   HeapSnapshotsCollection* snapshots_;
   unsigned next_snapshot_uid_;