How to decode “Content-Type: application/x-protobuffer”

Question

When I capture request from google captchas, I received this post data:

T9w1ROdplctW2nVKvNJYXH8o 
03AGdBq27xxQgyyA61mjxP_28XmqWkkFxE5zXbNUZMTT3I32sJrGLrNtf0w1