rAc: change how enabling/disabling Autofill affects requestAutocomplete().

Before, disabling Autofill would cause form.requestAutocomplete() to simply
dispatch an AutocompleteErrorEvent with a reason of "disabled". Now, disabling
Autofill and invoking rAc still shows the dialog with no ability to save or
display Autofill data. This is a step forward for web authors as they can use
our dialog with higher confidence of it always being there while privacy-centric
users can also be happy.

I should note that requestAutocomplete() still requires a user action to be
invoked, so a page will still need to do something like:

  button.onclick = function() { form.requestAutocomplete(); };

to invoke the dialog, which I believe will be mitigation for a majority of bad
actors attempting to misuse rAc.

BUG=342942
R=estade@chromium.org
TEST=unit_tests, chrome://settings/search#autofill, [ /x] Enable Autofill...
     while requestAutocomplete() dialog is showing

Review URL: https://codereview.chromium.org/145183004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252442 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252442