Small clean up of externalizing strings.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 7098859de3cfa9fa85d786b29cf8eef56846f086..304a8fa2eff64b818559e74f1fdfd6ebdd8e3537 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -5475,7 +5475,7 @@ Local<String> v8::String::NewExternal(
 bool v8::String::MakeExternal(v8::String::ExternalStringResource* resource) {
   i::Handle<i::String> obj = Utils::OpenHandle(this);
   i::Isolate* isolate = obj->GetIsolate();
-  if (i::StringShape(*obj).IsExternalTwoByte()) {
+  if (i::StringShape(*obj).IsExternal()) {
     return false;  // Already an external string.
   }
   ENTER_V8(isolate);
@@ -5488,6 +5488,8 @@ bool v8::String::MakeExternal(v8::String::ExternalStringResource* resource) {
   CHECK(resource && resource->data());
 
   bool result = obj->MakeExternal(resource);
+  // Assert that if CanMakeExternal(), then externalizing actually succeeds.
+  DCHECK(!CanMakeExternal() || result);
   if (result) {
     DCHECK(obj->IsExternalString());
     isolate->heap()->external_string_table()->AddString(*obj);
@@ -5515,7 +5517,7 @@ bool v8::String::MakeExternal(
     v8::String::ExternalAsciiStringResource* resource) {
   i::Handle<i::String> obj = Utils::OpenHandle(this);
   i::Isolate* isolate = obj->GetIsolate();
-  if (i::StringShape(*obj).IsExternalTwoByte()) {
+  if (i::StringShape(*obj).IsExternal()) {
     return false;  // Already an external string.
   }
   ENTER_V8(isolate);
@@ -5528,6 +5530,8 @@ bool v8::String::MakeExternal(
   CHECK(resource && resource->data());
 
   bool result = obj->MakeExternal(resource);
+  // Assert that if CanMakeExternal(), then externalizing actually succeeds.
+  DCHECK(!CanMakeExternal() || result);
   if (result) {
     DCHECK(obj->IsExternalString());
     isolate->heap()->external_string_table()->AddString(*obj);