In the latest statement from Coinbase :
In the event of two separate blockchains after August 1, 2017 we will only support one version. We have no plans to support the Bitcoin Cash fork. We have made this decision because it is hard to predict how long the alternative version of bitcoin will survive and if Bitcoin Cash will have future market value.
This means if there are two separate digital currencies — bitcoin (BTC) and bitcoin cash (BCC) — customers with Bitcoin stored on Coinbase will only have access to the current version of bitcoin we support (BTC). Customers will not have access to, or be able to withdraw, bitcoin cash (BCC).
Customers who wish to access both bitcoin (BTC) and bitcoin cash (BCC) need to withdraw bitcoin stored on Coinbase before 11.59 pm PT July 31, 2017. If you do not wish to access bitcoin cash (BCC) then no action is required.
If want to have BTC then Coinbase will be continuing to support and no need to worry.