Add support for ES6 Symbol in heap profiler

src/heap-snapshot-generator.h

Index: src/heap-snapshot-generator.h
diff --git a/src/heap-snapshot-generator.h b/src/heap-snapshot-generator.h
index a0f2a6293c11c042ba3242ae2d4fa1bf42ca653d..e1c291e90147f6b99dfc9bab2817e8a7c70ef4de 100644
--- a/src/heap-snapshot-generator.h
+++ b/src/heap-snapshot-generator.h
@@ -83,7 +83,8 @@ class HeapEntry BASE_EMBEDDED {
     kNative = v8::HeapGraphNode::kNative,
     kSynthetic = v8::HeapGraphNode::kSynthetic,
     kConsString = v8::HeapGraphNode::kConsString,
-    kSlicedString = v8::HeapGraphNode::kSlicedString
+    kSlicedString = v8::HeapGraphNode::kSlicedString,
+    kSymbol = v8::HeapGraphNode::kSymbol
   };
   static const int kNoEntry;
 
@@ -368,6 +369,7 @@ class V8HeapExplorer : public HeapEntriesAllocator {
   void ExtractJSGlobalProxyReferences(int entry, JSGlobalProxy* proxy);
   void ExtractJSObjectReferences(int entry, JSObject* js_obj);
   void ExtractStringReferences(int entry, String* obj);
+  void ExtractSymbolReferences(int entry, Symbol* symbol);
   void ExtractContextReferences(int entry, Context* context);
   void ExtractMapReferences(int entry, Map* map);
   void ExtractSharedFunctionInfoReferences(int entry,