Revert "Remove NOTIFICATION_WEB_CONTENTS_VISIBILITY_CHANGED."

This reverts r228081.

BUG=170921,308177

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228959 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228959