Add constant for native NTP URL.

BUG=178915
NOTRY=true

Review URL: https://codereview.chromium.org/62283004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233616 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233616