Autofilling passwords imported from IE more conservative

The trick is to leave action, username_element and password_element
fields of the form empty, this will stop the browser from autofilling
every form on the page with the credentials and will require user
interaction.
The user will have to click on the input box, select his login from
the drop down and click 'submit'. When this happens, Login Data will
be updated with the action url of the particular form.

Basically, since we don't know which form on the page is the one we 
have imported credentials for, we don't autofill anything until the 
user tells us. We'll remember his choice afterwards.

BUG=52513

Review URL: https://codereview.chromium.org/153043005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251035 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251035