Note: FIRST read this thread on XDA to determine which boot-to-root to download. As of today (2017-01-24), I used v2.78 SR3 for non-TWRP.
1. In Developer options, enable OEM unlocking
2. Unlock bootloader (this only works on Pixel directly bought from Google, not carrier versions):
(a) boot into bootloader mode:
adb reboot bootloader
fastboot flashing unlock
then confirm on phone
3. Download boot-to-root image from Chainfire (read this thread on XDA to determine the download version), then
fastboot boot boot-to-root.img
Note: after root, if Pixel is stuck in the booting process (e.g. because wrong version of boot-to-root.img is used), flash the factory boot.img from Google (fastboot flash boot boot.img), normal reboot, and repeat Step 3 above.