GitLab

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