Add an observer for article updates.

Define an observer that allows clients to listen to article updates.
In a subsequent CL, I will hook the webui to observe article changes.

BUG=288015

Review URL: https://codereview.chromium.org/100463005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239703 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239703