Only detect heuristic autofill types on page load (not on form submission).

      BUG=75862, 76034
TEST=none

Review URL: http://codereview.chromium.org/6673033

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@78336 0039d316-1c4b-4281-b951-d872f2087c98