Add a TurboFan skeleton for StringAddStub.

src/compiler/code-generator.h

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_COMPILER_CODE_GENERATOR_H_
 #define V8_COMPILER_CODE_GENERATOR_H_
 
 #include "src/compiler/gap-resolver.h"
 #include "src/compiler/instruction.h"
 #include "src/deoptimizer.h"
@@ skipping 166 matching lines @@
   GapResolver resolver_;
   SafepointTableBuilder safepoints_;
   ZoneVector<HandlerInfo> handlers_;
   ZoneDeque<DeoptimizationState*> deoptimization_states_;
   ZoneDeque<Handle<Object>> deoptimization_literals_;
   TranslationBuffer translations_;
   int last_lazy_deopt_pc_;
   JumpTable* jump_tables_;
   OutOfLineCode* ools_;
   int osr_pc_offset_;
+  bool needs_frame_;
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_CODE_GENERATOR_H