How can I set my latitude and longitude for debugging the Geolocation API with Google Chrome?

In Chrome today (version 42), open Developer Tools, click the "Toggle Device Icon", then in the "Emulation" drawer, chose "Sensors". There, you can Emulate geolocation coordinates and even "Emulate position unknown".

If you're talking about the Geolocation API, you can override the function:

navigator.geolocation.getCurrentPosition = function(success, failure) { 
    success({ coords: { 
        latitude: 30, 
        longitude: -105,

    }, timestamp: Date.now() }); 
} 

So when a library calls into the navigator.geolocation.getCurrentPosition function the coordinates you specify will be returned.

A little late on the answer, but in Chrome you can open the Developer Tools (F12 or Ctrl + Shift + I). In the lower right hand open the 'Settings' gear and switch to the 'Overrides' tab. There is a checkbox labeled 'Override Geolocation'. Check this box and enter in what ever geolocation that you want the site to think you are at.

In case I haven't explained it clearly enough, here is a good article that goes over it with screenshots and what not: http://www.labnol.org/internet/geo-location/27878/