Add Box object to heap profiler.

src/heap-snapshot-generator.cc

Index: src/heap-snapshot-generator.cc
diff --git a/src/heap-snapshot-generator.cc b/src/heap-snapshot-generator.cc
index 9f4d6da89ffec82258151eda30dd9c62532026b5..ccfbfb8d03af5757c67dfc27f3dc2fc073357931 100644
--- a/src/heap-snapshot-generator.cc
+++ b/src/heap-snapshot-generator.cc
@@ -1047,6 +1047,8 @@ void V8HeapExplorer::ExtractReferences(HeapObject* obj) {
     ExtractCodeCacheReferences(entry, CodeCache::cast(obj));
   } else if (obj->IsCode()) {
     ExtractCodeReferences(entry, Code::cast(obj));
+  } else if (obj->IsBox()) {
+    ExtractBoxReferences(entry, Box::cast(obj));
   } else if (obj->IsCell()) {
     ExtractCellReferences(entry, Cell::cast(obj));
   } else if (obj->IsPropertyCell()) {
@@ -1417,6 +1419,11 @@ void V8HeapExplorer::ExtractCodeReferences(int entry, Code* code) {
 }
 
 
+void V8HeapExplorer::ExtractBoxReferences(int entry, Box* box) {
+  SetInternalReference(box, entry, "value", box->value(), Box::kValueOffset);
+}
+
+
 void V8HeapExplorer::ExtractCellReferences(int entry, Cell* cell) {
   SetInternalReference(cell, entry, "value", cell->value(), Cell::kValueOffset);
 }