Add a TurboFan skeleton for StringAddStub.

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

Index: src/compiler/mips/code-generator-mips.cc
diff --git a/src/compiler/mips/code-generator-mips.cc b/src/compiler/mips/code-generator-mips.cc
index 04611903cf46dca12afbafb2287a180637b96c55..6e2c4e244c72e74b1ff11e5883a713584d6e4003 100644
--- a/src/compiler/mips/code-generator-mips.cc
+++ b/src/compiler/mips/code-generator-mips.cc
@@ -1075,7 +1075,7 @@ void CodeGenerator::AssemblePrologue() {
     __ Prologue(info->IsCodePreAgingActive());
     frame()->SetRegisterSaveAreaSize(
         StandardFrameConstants::kFixedFrameSizeFromFp);
-  } else if (stack_slots > 0) {
+  } else if (needs_frame_) {
     __ StubPrologue();
     frame()->SetRegisterSaveAreaSize(
         StandardFrameConstants::kFixedFrameSizeFromFp);
@@ -1121,7 +1121,7 @@ void CodeGenerator::AssembleReturn() {
     __ mov(sp, fp);
     __ Pop(ra, fp);
     __ Ret();
-  } else if (descriptor->IsJSFunctionCall() || stack_slots > 0) {
+  } else if (descriptor->IsJSFunctionCall() || needs_frame_) {
     __ mov(sp, fp);
     __ Pop(ra, fp);
     int pop_count = descriptor->IsJSFunctionCall()