Added flag enabling new first-run UI to about:flags.

BUG=269294
NOTRY=true
TBR=nkostylev

Review URL: https://codereview.chromium.org/25099002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225723 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225723