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

src/arm/lithium-codegen-arm.cc

Index: src/arm/lithium-codegen-arm.cc
diff --git a/src/arm/lithium-codegen-arm.cc b/src/arm/lithium-codegen-arm.cc
index 9900a1e9848f0515b093bb423c4b212de5e81fb2..d46de82494e74eb8f5b8ee1ced96b4a63deb52eb 100644
--- a/src/arm/lithium-codegen-arm.cc
+++ b/src/arm/lithium-codegen-arm.cc
@@ -1041,11 +1041,6 @@ void LCodeGen::DoCallStub(LCallStub* instr) {
       CallCode(stub.GetCode(isolate()), RelocInfo::CODE_TARGET, instr);
       break;
     }
-    case CodeStub::StringAdd: {
-      StringAddStub stub(NO_STRING_ADD_FLAGS);
-      CallCode(stub.GetCode(isolate()), RelocInfo::CODE_TARGET, instr);
-      break;
-    }
     case CodeStub::StringCompare: {
       StringCompareStub stub;
       CallCode(stub.GetCode(isolate()), RelocInfo::CODE_TARGET, instr);
@@ -4539,7 +4534,7 @@ void LCodeGen::DoTrapAllocationMemento(LTrapAllocationMemento* instr) {
 void LCodeGen::DoStringAdd(LStringAdd* instr) {
   __ push(ToRegister(instr->left()));
   __ push(ToRegister(instr->right()));
-  StringAddStub stub(NO_STRING_CHECK_IN_STUB);
+  StringAddStub stub(instr->hydrogen()->flags());
   CallCode(stub.GetCode(isolate()), RelocInfo::CODE_TARGET, instr);
 }