The assistant iframe is rendered without style

I'm using Firefox 57.0b3, and on it, sometimes the assistant ui frame is rendered before styles are applied.
Attached a screencast demonstrating the issue: [1.zip](https://github.com/AdguardTeam/AdguardAssistant/files/1337703/1.zip)
