Only load text fields in the form structure.  This prevents us from trying to autofill username/password forms.

BUG=none
TEST=FormStructureTest
TEST=log in to gmail.  AutoFill infobar should not show up.
Review URL: http://codereview.chromium.org/661026

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@39985 0039d316-1c4b-4281-b951-d872f2087c98