Add a TurboFan skeleton for StringAddStub.

src/globals.h

Index: src/globals.h
diff --git a/src/globals.h b/src/globals.h
index 761983178d4b0f4d2f316744eed9d4ff818474e4..00665e3ffdde2dcd3b0b6d16d2d3af4594aaf018 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -240,7 +240,7 @@ enum LanguageMode {
 };
 
 
-inline std::ostream& operator<<(std::ostream& os, LanguageMode mode) {
+inline std::ostream& operator<<(std::ostream& os, const LanguageMode& mode) {
   switch (mode) {
     case SLOPPY:
       return os << "sloppy";
@@ -446,6 +446,17 @@ enum AllocationAlignment { kWordAligned, kDoubleAligned, kDoubleUnaligned };
 // allows).
 enum PretenureFlag { NOT_TENURED, TENURED };
 
+inline std::ostream& operator<<(std::ostream& os, const PretenureFlag& flag) {
+  switch (flag) {
+    case NOT_TENURED:
+      return os << "NotTenured";
+    case TENURED:
+      return os << "Tenured";
+  }
+  UNREACHABLE();
+  return os;
+}
+
 enum MinimumCapacity {
   USE_DEFAULT_MINIMUM_CAPACITY,
   USE_CUSTOM_MINIMUM_CAPACITY