[Autofill] Refactor FillFormField to be shared between local Autofill and Wallet.

BUG=263705
TEST=unit_tests
R=estade@chromium.org

Review URL: https://codereview.chromium.org/26320010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230359 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230359