What happens to my Bitcoins if I leave them at Coinbase after the hard fork?
This is what Coinbase says about the November 2017 expected hard fork:
Customers with bitcoin balances stored on Coinbase at the time of the fork will have access to bitcoin on both blockchains. There is no action required from customers and bitcoin can be securely stored on Coinbase before, during, and after the fork. Read more about it here: 
https://blog.coinbase.com/update-on-the-bitcoin-segwit2x-hard-fork-69426f14bc85