Convert Google Now's Authentication Manager to use Promises

BUG=164227
R=rgustafson@chromium.org, skare@chromium.org, vadimt@chromium.org

Review URL: https://codereview.chromium.org/162273002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252109 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252109