Update comment for ShouldForceOnlineSignIn() conditions.

BUG=346308
NOTRY=true

Review URL: https://codereview.chromium.org/179613003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253131 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253131