[stubs] Port SubStringStub to TurboFan

src/code-stubs.cc

Index: src/code-stubs.cc
diff --git a/src/code-stubs.cc b/src/code-stubs.cc
index e74d408ca62905c41c13d672ab5b22eb53fdf193..c5fc445af1102eeb69b58f1f3a69b80cfadd519d 100644
--- a/src/code-stubs.cc
+++ b/src/code-stubs.cc
@@ -2688,6 +2688,15 @@ compiler::Node* DecStub::Generate(CodeStubAssembler* assembler,
   return result_var.value();
 }
 
+// TODO(jgruber): Comment.
+compiler::Node* SubStringStub::Generate(CodeStubAssembler* assembler,
+                                        compiler::Node* string,
+                                        compiler::Node* from,
+                                        compiler::Node* to,
+                                        compiler::Node* context) {
+  return assembler->StringSubString(context, string, from, to);
+}
+
 // ES6 section 7.1.13 ToObject (argument)
 void ToObjectStub::GenerateAssembly(CodeStubAssembler* assembler) const {
   typedef compiler::Node Node;