Fix many issues with the CVV challenge code path.

So far I've fixed the:
- height issue while showing the challenge
- generated card bubble DCHECK() from an empty backing card
- validation issues during the challenge

Still need to:
- clarify what sets the height of the overlay while sending the CVV

BUG=306304
R=estade@chromium.org

Review URL: https://codereview.chromium.org/27415006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229736 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229736