Change cast to String to cast to Symbol in heap-snapshot-generator.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/684893002

Cr-Commit-Position: refs/heads/master@{#24959}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24959 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/heap-snapshot-generator.cc

@@ -1698,7 +1698,7 @@ void V8HeapExplorer::ExtractPropertyReferences(JSObject* js_obj, int entry) {
           continue;
         }
         if (ExtractAccessorPairProperty(js_obj, entry, k, value)) continue;
-        SetPropertyReference(js_obj, entry, String::cast(k), value);
+        SetPropertyReference(js_obj, entry, Name::cast(k), value);
       }
     }
   }