Make kChromeSigninEffectiveSite const-correct.

BUG=none
TEST=none
R=rogerta@chromium.org

Review URL: https://codereview.chromium.org/196553004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256724 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256724