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The Google Pixel comes with its own USB Power Delivery based charger that supports 15 watts (5 volts @ 3 amps) and 18 watts (9 volts @ 2 amps). 

Using software from Granite River Labs, we see a very clean and simple USB Power Delivery negotiation when the battery is well charged. The Google 18W USB-C Power Adapter simply advertises it can deliver 15 watts (5 volts @ 3 amps) and 18 watts (9 volts @ 2 amps), and the Google Pixel subsequently requests to be charged at 10 watts (5 volts @ 2 amps).  

We observed that this charger actually delivers about 1.5 amps @ 5 volts to the Google Pixel.

It's interesting that the Google Pixel didn't take advantage of the higher power available but we'll be taking a closer look to see if there is a more complex power negotiation going on.