Rename CheckedRadioButtons to RadioButtonGroupScope.

This CL fixes a FIXME on CheckedRadioButtons class.

BUG=none
TEST=none; no behavior changes

Review URL: https://codereview.chromium.org/169233002

git-svn-id: svn://svn.chromium.org/blink/trunk@167268 bbb929c8-8fbe-4397-9dbb-9b2b20218538