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Get some #0000 steel wool and dip a wad in chrome cleaner/polish such as Mother's and work it into the burn rubber mark. Let is sit a few minutes and repeat. It should come right off without leaving any scratches. I have used this method many times and have not had any problems with scratching the chrome. Some people has also had success with spray on oven cleaner, but I have never tried it.
oven cleaner works great, just don't get it on anything else ( paint ,rubber, etc) hold your rag away from your bike and spray the oven cleaner onto the rag, then apply to the "boot melt"