If these are booked separately, it's likely you'll be charged. If booked on the same ticket, do they have an inter line agreement? And, with the IATA agreement, the most significant carrier (Air China as it's the long transpacific leg) should determine the baggage terms.

Depending on when you plan to do this, AA will start PEK-DFW flights with a 777 in May (in June, it is served by a 787).