Fix OAuth tutorial by adding missing link/mention of onload.js.

BUG=306867
R=kalman@chromium.org, mek@chromium.org

Review URL: https://codereview.chromium.org/26554005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229041 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229041