Add a TurboFan skeleton for StringAddStub.

src/compiler/instruction.h

Index: src/compiler/instruction.h
diff --git a/src/compiler/instruction.h b/src/compiler/instruction.h
index e9cd4d6d0867b24b386f65b0e7f95feccfc9f14c..10273a495c9f01870ed0f0e8eec06b6a433d9ba0 100644
--- a/src/compiler/instruction.h
+++ b/src/compiler/instruction.h
@@ -1156,6 +1156,13 @@ class InstructionSequence final : public ZoneObject {
                          SourcePosition* result) const;
   void SetSourcePosition(const Instruction* instr, SourcePosition value);
 
+  bool ContainsCall() const {
+    for (Instruction* instr : instructions_) {
+      if (instr->IsCall()) return true;
+    }
+    return false;
+  }
+
  private:
   friend std::ostream& operator<<(std::ostream& os,
                                   const PrintableInstructionSequence& code);