I think that the atypical combination of art history and chemistry as prospective majors actually lends distinction to your application.
Rather than regarding it as a weakness that signifies a confusing dilution of commitment to any one field, I see it as a strength, as a quasi-hook . . . assuming, of course, that you can (as the above poster suggests) utilize your essay to link your interest in those disparate fields into a coherent narrative that serves to project a clearer image of who you are as a person and what you hope both to learn from and contribute to the school[s] to which you are applying.
In terms of chances, you look solid for all the UCs save UCLA and UCB, and even at those two you have considerably better-than-average odds. You also look good at UT. Your well rounded ECs make you a solid candidate at Amherst–though that reading score is a bit low for a top LAC (and Amherst remains a reach for everyone). For the Ivies, MIT, Stanford, and Rice, you will greatly improve your chances if you retake the SAT and raise the reading score.
A final note: Given your interest in art history, Williams seems like an obvious target among the elite LACs.