Move common functionality to a base class MessageView.
This includes background_view and ::Layout().

BUG=320827

Review URL: https://codereview.chromium.org/114813004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241267 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241267