ChromeOS Photos app supports playing videos with external subtitles, but there are two conditions for that:

1. The subtitle file should be in the WebVTT format (with .vtt extension)
2. The subtitle file should have the same name as the video file, except the extension.

Once the subtitle file satisfies those conditions, open it with the Photos app and you will have an option to turn on the captions in the menu.

If the subtitle for your video is in the SubRip format (with .srt extension), you can convert it to .vtt using: http://www.webvtt.org