Consistently use HStringAdd instead of HCallStub with CodeStub::StringAdd.

src/hydrogen-instructions.cc

Index: src/hydrogen-instructions.cc
diff --git a/src/hydrogen-instructions.cc b/src/hydrogen-instructions.cc
index 16f502ddaf7b11f3d9600ecd40b025dc899e38d1..bd7beecb66979987e489a98d31366de7ec1b3445 100644
--- a/src/hydrogen-instructions.cc
+++ b/src/hydrogen-instructions.cc
@@ -3451,8 +3451,11 @@ DEFINE_NEW_H_SIMPLE_ARITHMETIC_INSTR(HSub, -)
 #undef DEFINE_NEW_H_SIMPLE_ARITHMETIC_INSTR
 
 
-HInstruction* HStringAdd::New(
-    Zone* zone, HValue* context, HValue* left, HValue* right) {
+HInstruction* HStringAdd::New(Zone* zone,
+                              HValue* context,
+                              HValue* left,
+                              HValue* right,
+                              StringAddFlags flags) {
   if (FLAG_fold_constants && left->IsConstant() && right->IsConstant()) {
     HConstant* c_right = HConstant::cast(right);
     HConstant* c_left = HConstant::cast(left);
@@ -3462,7 +3465,7 @@ HInstruction* HStringAdd::New(
       return new(zone) HConstant(concat, Representation::Tagged());
     }
   }
-  return new(zone) HStringAdd(context, left, right);
+  return new(zone) HStringAdd(context, left, right, flags);
 }