[runtime] Replace hidden_string with a 0-hash-code private symbol

src/profiler/heap-snapshot-generator.cc

Index: src/profiler/heap-snapshot-generator.cc
diff --git a/src/profiler/heap-snapshot-generator.cc b/src/profiler/heap-snapshot-generator.cc
index f228dd5055a76f36e6ca76a1786e8c17efd5c4a5..85ad029ecb83b2b0ca0118e305b5c76336b44fef 100644
--- a/src/profiler/heap-snapshot-generator.cc
+++ b/src/profiler/heap-snapshot-generator.cc
@@ -1598,7 +1598,7 @@ void V8HeapExplorer::ExtractPropertyReferences(JSObject* js_obj, int entry) {
           int field_offset =
               field_index.is_inobject() ? field_index.offset() : -1;
 
-          if (k != heap_->hidden_string()) {
+          if (k != heap_->hidden_properties_symbol()) {
             SetDataOrAccessorPropertyReference(details.kind(), js_obj, entry, k,
                                                value, NULL, field_offset);
           } else {
@@ -1625,7 +1625,7 @@ void V8HeapExplorer::ExtractPropertyReferences(JSObject* js_obj, int entry) {
         DCHECK(dictionary->ValueAt(i)->IsPropertyCell());
         PropertyCell* cell = PropertyCell::cast(dictionary->ValueAt(i));
         Object* value = cell->value();
-        if (k == heap_->hidden_string()) {
+        if (k == heap_->hidden_properties_symbol()) {
           TagObject(value, "(hidden properties)");
           SetInternalReference(js_obj, entry, "hidden_properties", value);
           continue;
@@ -1642,7 +1642,7 @@ void V8HeapExplorer::ExtractPropertyReferences(JSObject* js_obj, int entry) {
       Object* k = dictionary->KeyAt(i);
       if (dictionary->IsKey(k)) {
         Object* value = dictionary->ValueAt(i);
-        if (k == heap_->hidden_string()) {
+        if (k == heap_->hidden_properties_symbol()) {
           TagObject(value, "(hidden properties)");
           SetInternalReference(js_obj, entry, "hidden_properties", value);
           continue;