Add FPS metric to the extension

[callicles]

Right now the FPS areas in the extension are placeholders, they should be implemented.

[callicles]

We have to wait for this:
https://code.google.com/p/chromium/issues/detail?id=120796

To happen, thus it is out for now.

[callicles]

Temp solution could be to do something like this: https://github.com/okor/justice with requestAnimationFrame. But that's not that clean