Took me a while to fix this problem so thought this may help with Q&A. Trying to debug Android app on my new Nexus 5. Able to enable developer mode in normal manner but not visible in Chrome when connected by USB using a USB cable that had a Micro USB to Type C converter.
USB Type C converter seemed to only support charging. I bought a USB cable with Type C end point and this solved the problem. Phone was now visible in Chrome.