ES6 symbols: Introduce Symbol class, along with abstract Name class

src/objects-visiting.cc

Index: src/objects-visiting.cc
diff --git a/src/objects-visiting.cc b/src/objects-visiting.cc
index 7082e59f3588a1c32075a82b29a5f924556de518..088f5ebdeb354a879da1f519e9a5ba08d9381fee 100644
--- a/src/objects-visiting.cc
+++ b/src/objects-visiting.cc
@@ -128,6 +128,11 @@ StaticVisitorBase::VisitorId StaticVisitorBase::GetVisitorId(
                                  kVisitDataObjectGeneric,
                                  Foreign::kSize);
 
+    case SYMBOL_TYPE:
+      return GetVisitorIdForSize(kVisitDataObject,
+                                 kVisitDataObjectGeneric,
+                                 Symbol::kSize);
+
     case FILLER_TYPE:
       return kVisitDataObjectGeneric;