Fixes navigation issues with interstitial pages.

      Updates NavigationController so that the back and forward menus can
be used while an interstitial page is showing.  Also fixes tab cloning
while an interstitial page is showing, so that the cloned tab does not
have the interstitial navigation entry.

BUG=37215
BUG=37894
TEST=SSLUITest.TestHTTPSExpiredCertAndGo{Back,Forward}
TEST=NavigationControllerTest.CloneOmitsInterstitials

Review URL: http://codereview.chromium.org/861001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@41304 0039d316-1c4b-4281-b951-d872f2087c98