rAc: split TestableAutofillDialogView implementation into its own class

This:
- makes use of a funky downcast
- moves TestableAutofillDialogViews code out of the production binary
- makes it impossible to accidentally call testing code from production code

mac implementation by groby@chromium.org

BUG=306200

Review URL: https://codereview.chromium.org/135933003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247886 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247886
Automated Commit: Committing new LKGM version 5384.0.0 for chromeos.

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247887 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247887