Eliminate AutofillManager exposing public accessor for WebContents.

Limit AutofillManager's knowledge of WebContents to being private as we work to
abstract this knowledge altogether.

BUG=247015

Review URL: https://codereview.chromium.org/25492002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226253 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226253