The RobotDrive#setInvertedMotor() method takes a RobotDrive.MotorType object as the first parameter instead of a reference to the actual motor. Try using this:
Code:
...
chassis.setInvertedMotor(RobotDrive.MotorType.kFrontLeft, true);
chassis.setInvertedMotor(RobotDrive.MotorType.kRearLeft, true);
...
Also, don't forget that the joysticks treat forward motion along the X axis as a negative value, whereas the RobotDrive treats a positive value as forwards movement (I think; could someone else confirm this? It may depend on your setup). You may want to negate your X axis reading as well.