[runtime] Minor cleanup of inferred-name handling

Bug: 
Change-Id: Iccc0241859970ad4e04b9a6e5f206491aa7c442e
Reviewed-on: https://chromium-review.googlesource.com/781939Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49541}

src/objects.cc

@@ -3707,8 +3707,7 @@ Handle<String> JSReceiver::GetConstructorName(Handle<JSReceiver> receiver) {
     Object* maybe_constructor = receiver->map()->GetConstructor();
     if (maybe_constructor->IsJSFunction()) {
       JSFunction* constructor = JSFunction::cast(maybe_constructor);
-      String* name = constructor->shared()->name();
-      if (name->length() == 0) name = constructor->shared()->inferred_name();
+      String* name = constructor->shared()->DebugName();
       if (name->length() != 0 &&
           !name->Equals(isolate->heap()->Object_string())) {
         return handle(name, isolate);
@@ -3735,8 +3734,7 @@ Handle<String> JSReceiver::GetConstructorName(Handle<JSReceiver> receiver) {
   Handle<String> result = isolate->factory()->Object_string();
   if (maybe_constructor->IsJSFunction()) {
     JSFunction* constructor = JSFunction::cast(*maybe_constructor);
-    String* name = constructor->shared()->name();
-    if (name->length() == 0) name = constructor->shared()->inferred_name();
+    String* name = constructor->shared()->DebugName();
     if (name->length() > 0) result = handle(name, isolate);
   }
 
@@ -13572,9 +13570,8 @@ void SharedFunctionInfo::set_debugger_hints(int value) {
 }
 
 String* SharedFunctionInfo::DebugName() {
-  String* n = name();
-  if (String::cast(n)->length() == 0) return inferred_name();
-  return String::cast(n);
+  if (name()->length() == 0) return inferred_name();
+  return name();
 }
 
 bool SharedFunctionInfo::HasNoSideEffect() {

src/objects/shared-function-info-inl.h

@@ -362,9 +362,9 @@ String* SharedFunctionInfo::inferred_name() {
   if (HasInferredName()) {
     return String::cast(function_identifier());
   }
-  Isolate* isolate = GetIsolate();
-  DCHECK(function_identifier()->IsUndefined(isolate) || HasBuiltinFunctionId());
-  return isolate->heap()->empty_string();
+  DCHECK(function_identifier()->IsUndefined(GetIsolate()) ||
+         HasBuiltinFunctionId());
+  return GetHeap()->empty_string();
 }
 
 void SharedFunctionInfo::set_inferred_name(String* inferred_name) {