Allow empty iframe tiles in the NTP and log them to UMA.

These empty iframes allow controlling the visual of the tiles externally, from the page itself, rather than within the iframe.

BUG=323186

Review URL: https://codereview.chromium.org/85953003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238795 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238795