The June Pixel Feature Drop has added a new feature to automatically start the timer using the palm of your hand. To activate it, you need to update the Google Camera app to version 8.9.097.538010547.31. Here is how to use it:
1. Open the camera
2. In Quick settings menu select timer mode (for 3 or 10 seconds)
3. Raise your palm to activate and wait until the square fills up, then the timer is activated.
This function also works on the rear camera.