rAc: don't open sign in continue url in new tab

BUG=326882,331127
TEST=rac sign in works

Review URL: https://codereview.chromium.org/123363002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243030 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243030