The option is accessible as Sound & notification in the main Android settings. The same option can be used to turn on and turn off the pulse notification light. If you were wondering, the light is present in the front part of the Nexus 6P phone, right next to the front camera towards the top left of the phone itself. This is a hardware feature.
To make use of this feature:
- Open Settings from the app drawer.
- Scroll down to Device and under it tap on Sound & notification.
- Scroll a bit down and under Notification > Pulse notification light move the slider towards the right.