scrolledwindow: Bug 766569 - General size request fixes.

scrolledwindow: Bug 766569 - General size request fixes.

This patch does a couple of things:

  o Removes the obscure 'extra_width' and 'extra_height' variables
    making the request code exceedingly difficult to read

  o Fixes the max-content-size properties introduced in bug 742281
    so that they do not grow the minimum request.

  o Cleanup of request code in general:
    - min/max content sizes are clamped around the child request as needed
    - scrollbar requests are only added in one place, after child request
      sizes are calculated and without the extra_width/height thing.