Extract internal classes into separate message_center namespace for eventual move to ui/.

TBR for ash OWNERS.

BUG=150872
[email protected]

Review URL: https://chromiumcodereview.appspot.com/10960022

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@159039 0039d316-1c4b-4281-b951-d872f2087c98
17 files changed