According to coinbase they are currently experiencing transaction delays. Quoting it below:
Identified - Due to high volume, we are experiencing a backlog of outgoing transactions for BTC and ETH. Our team is working on increasing our capacity so that we can process transactions more quickly. Sends of BTC and ETH may be delayed by several hours. We apologize for any inconvenience this may cause.
Dec 26, 10:24 PST
There was a guy with a similar problem. I think his transaction got processed in less than 12 hours or 24 hours. So you just need to wait, if 24 hours has passed and it's still pending, you should then contact their support.
Here's the thread link of the guy I was talking about:
https://bitcointalk.org/index.php?topic=2646631