What I did when my Nexus 6P didn´t boot past Google logo and entered in a loop that never passed that state, was:
First, I entered in the Bootload section pressing at the same time the Power button + the volume-down button.
Then, pressing twice the volumen-down button I chose the Recovery-Mode option. (You confirm it by pressing the Power button)
After that, an Android icon with an exclamation sign logo will appear and below this a statement that says "no commands" or something like that.
Now, to enter the Recovery-Mode you have to press, not at the same time, but first the Power button and without releasing it press the VOLUME-UP button, and a screen with many options will appear.
I selected the "Wipe Data/Factory reset" option and when it finished, I turned the phone off.
Then I turned it on but it kept booting and rebooting never passing the Google logo, so I left it on my desk rebooting and went to my job. When I came back, after a few hours, the phone was perfect and had the screen they have when you for the first time turn it on and says something like "Welcome, choose language".
Summary, reset it to Factory and turn it off. After that turn it on and go for a walk or whatever but leave it rebooting a couple of hours. It worked for me and I hope it can work for other people too.