what is it that julia takes for granted?

This can be one of two answers based on the way you've asked the question;

First, Julia presumes that her conversations in the small room above Mr. Charrington's shop are safe. While in the room, she believes that her meeting and conversations with Winston are safe from discovery.

Second, Julia accepts all of the "BIg Brother," policies, principles, and decisons wihtout question. She doesn't question them, and therefore takes it for granted that they're doing the right things.

I think she takes for granted that everyone hates BB.