Return MaybeHandle from NewExternalStringFrom*.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 1386a3a0a2b7787dbcf2c71e6ca0d49ffbe6faac..c319c3aa5dfd4f0406d2304f848998765d95f6fd 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -5470,22 +5470,18 @@ Local<String> v8::String::Concat(Handle<String> left, Handle<String> right) {
 static i::Handle<i::String> NewExternalStringHandle(
     i::Isolate* isolate,
     v8::String::ExternalStringResource* resource) {
-  i::Handle<i::String> result =
-      isolate->factory()->NewExternalStringFromTwoByte(resource);
   // We do not expect this to fail. Change this if it does.
-  CHECK(!result.is_null());
-  return result;
+  return isolate->factory()->NewExternalStringFromTwoByte(
+      resource).ToHandleChecked();
 }
 
 
 static i::Handle<i::String> NewExternalAsciiStringHandle(
     i::Isolate* isolate,
     v8::String::ExternalAsciiStringResource* resource) {
-  i::Handle<i::String> result =
-      isolate->factory()->NewExternalStringFromAscii(resource);
   // We do not expect this to fail. Change this if it does.
-  CHECK(!result.is_null());
-  return result;
+  return isolate->factory()->NewExternalStringFromAscii(
+      resource).ToHandleChecked();
 }