Motion and shape tracking in After Effects CS5.5
I would suggest as a starting point to use Corner Pin Tracking, found on the tracking panel of AE.

Track each side of the sunglasses separately and when done, you should have two solids that move with the area of the glasses. Add a mask to each one, and rotoscope the actual shape of the glasses.

I think this would be easier than rotoscoping from scratch. These two solids can then be used as track mattes or precomposed etc.