Minor fixes in form_based_credentials_backend.py

BUG=NONE
TEST=unittest

Review URL: https://chromiumcodereview.appspot.com/23498048

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224772 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224772