Fix crasher with autofill infobar.

      The &quotsave credit card&quot information was deleted by the AutofillManager when the form is submitted but was still pointed to by the TabContents, causing a crasher when navigating.

BUG=50428
TEST=See steps in 56238.

Review URL: http://codereview.chromium.org/3466006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@60095 0039d316-1c4b-4281-b951-d872f2087c98