Rather than coupling this to the ```img_tiles``` implementation, how about adding ```cartopy.crs.GoogleMercator()``` and having ```img_tiles``` and this use that?
```python
class GoogleMercator(Mercator):
    def __init__(self, central_longitude=0.0):
        globe = ccrs.Globe(ellipse=None,
                           semimajor_axis=6378137,
                           semiminor_axis=6378137,
                           nadgrids='@null')
        super(GoogleMercator, self).__init__(central_longitude=central_longitude,
                                             min_latitude=-85.0511287798066,
                                             max_latitude=85.0511287798066,
                                             globe=globe)
```