Add a TurboFan skeleton for StringAddStub.

src/compiler/x64/code-generator-x64.cc

Index: src/compiler/x64/code-generator-x64.cc
diff --git a/src/compiler/x64/code-generator-x64.cc b/src/compiler/x64/code-generator-x64.cc
index 0f1e959adc7efd0cdacf20c38255032cdfa3cae0..2105d66e4714ba709efc1c1efca36ef31625bf1a 100644
--- a/src/compiler/x64/code-generator-x64.cc
+++ b/src/compiler/x64/code-generator-x64.cc
@@ -1451,7 +1451,7 @@ void CodeGenerator::AssemblePrologue() {
     __ Prologue(info->IsCodePreAgingActive());
     frame()->SetRegisterSaveAreaSize(
         StandardFrameConstants::kFixedFrameSizeFromFp);
-  } else if (stack_slots > 0) {
+  } else if (needs_frame_) {
     __ StubPrologue();
     frame()->SetRegisterSaveAreaSize(
         StandardFrameConstants::kFixedFrameSizeFromFp);
@@ -1504,7 +1504,7 @@ void CodeGenerator::AssembleReturn() {
       __ popq(rbp);       // Pop caller's frame pointer.
       __ ret(0);
     }
-  } else if (descriptor->IsJSFunctionCall() || stack_slots > 0) {
+  } else if (descriptor->IsJSFunctionCall() || needs_frame_) {
     __ movq(rsp, rbp);  // Move stack pointer back to frame pointer.
     __ popq(rbp);       // Pop caller's frame pointer.
     int pop_count = descriptor->IsJSFunctionCall()