NTP: add histogram to track New Tab page URL status.

Log status of the New Tab page URL (e.g. valid URL from search provider,
insecure url, etc.) whenever a New Tab is opened.

BUG=none

Review URL: https://codereview.chromium.org/150033002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248573 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248573