[Password Autofill] Don't prompt if the password value is default

Some sites have a placeholder value in the password field on page load. Now that we try saving all filled out forms on navigation, we need to ignore those that the user didn't interact with.

BUG=338650

Review URL: https://codereview.chromium.org/174423004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253704 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253704