Fix issues with supervised user recreation.

Issues fixed in this CL:
- Support high-resolution screens when showing the avatar icons.
- Disable import button when there are no supervised users at all to import.
- Don't overwrite the synced avatar on ChromeOS.
- Improve css properties in the import overlay.

A screenshot of the changed import dialog can be found in the bug description of 282464.

BUG=282464,314605

Review URL: https://codereview.chromium.org/100073004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240091 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240091