Add a TurboFan skeleton for StringAddStub.

src/compiler/ia32/code-generator-ia32.cc

Index: src/compiler/ia32/code-generator-ia32.cc
diff --git a/src/compiler/ia32/code-generator-ia32.cc b/src/compiler/ia32/code-generator-ia32.cc
index 0262c2a732325464b11a2e84bb08db88e52edbba..de9ac2e819430b7e14c19c6eac23152e88c8bb8d 100644
--- a/src/compiler/ia32/code-generator-ia32.cc
+++ b/src/compiler/ia32/code-generator-ia32.cc
@@ -1256,7 +1256,7 @@ void CodeGenerator::AssemblePrologue() {
     __ Prologue(info->IsCodePreAgingActive());
     frame()->SetRegisterSaveAreaSize(
         StandardFrameConstants::kFixedFrameSizeFromFp);
-  } else if (stack_slots > 0) {
+  } else if (needs_frame_) {
     __ StubPrologue();
     frame()->SetRegisterSaveAreaSize(
         StandardFrameConstants::kFixedFrameSizeFromFp);
@@ -1310,7 +1310,7 @@ void CodeGenerator::AssembleReturn() {
       __ pop(ebp);       // Pop caller's frame pointer.
       __ ret(0);
     }
-  } else if (descriptor->IsJSFunctionCall() || stack_slots > 0) {
+  } else if (descriptor->IsJSFunctionCall() || needs_frame_) {
     __ mov(esp, ebp);  // Move stack pointer back to frame pointer.
     __ pop(ebp);       // Pop caller's frame pointer.
     int pop_count = descriptor->IsJSFunctionCall()