Allow showing progress notification update in message center on Mac
Whether to show updates in the message center is controlled by the platform-independent layer. We do not need the logic in [MCTrayViewController onMessageCenterTrayChanged]. Remove it such that the progress update could be performed.
BUG=170924
TEST=Manual test by creating progress notifications and opening message center
Review URL: https://chromiumcodereview.appspot.com/
22311003
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@215759 0039d316-1c4b-4281-b951-d872f2087c98