[es2015] Precompute the descriptive string for symbols.

src/builtins/builtins-symbol-gen.cc

Index: src/builtins/builtins-symbol-gen.cc
diff --git a/src/builtins/builtins-symbol-gen.cc b/src/builtins/builtins-symbol-gen.cc
index 4015a2dcbab0fad2ea88faa54d1bb8471a09e4b4..6c5643e1f672cce82cc78c27c0569222bf7e5dce 100644
--- a/src/builtins/builtins-symbol-gen.cc
+++ b/src/builtins/builtins-symbol-gen.cc
@@ -29,7 +29,7 @@ TF_BUILTIN(SymbolPrototypeToString, CodeStubAssembler) {
 
   Node* value = ToThisValue(context, receiver, PrimitiveType::kSymbol,
                             "Symbol.prototype.toString");
-  Node* result = CallRuntime(Runtime::kSymbolDescriptiveString, context, value);
+  Node* result = LoadObjectField(value, Symbol::kDescriptiveStringOffset);
   Return(result);
 }