Add kEnhancedPlaybackNotificationLearnMoreURL.

This is used for the notification dialog of enhanced playback on ChromeOS.

BUG=340894
R=bartfab@chromium.org, mukai@chromium.org

Review URL: https://codereview.chromium.org/151473003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249709 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249709