Replace a stale fixme with a comment

Eliminate a stale FIXME comment in the SizesAttributeParser, and replaces it with a comment stating why sizes' media conditions should not be evaluated as non-screen media types.

BUG=357586
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/284263005

git-svn-id: svn://svn.chromium.org/blink/trunk@174206 bbb929c8-8fbe-4397-9dbb-9b2b20218538