Fix an out-of-date issue in chrome.webNavigation document.

The transitionType "start_page" is out of date, replace it with
"auto_toplevel"

BUG=333878

Review URL: https://codereview.chromium.org/137833002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247663 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247663