Adds a flag to control the notification center menu bar behavior on Mac.
Currently default behavior is to show the bell after the first
notification is created, until Chrome exits. This will remain the
default, but we will add 3 more states:
* Always show the bell
* Never show the bell (notifications will always toast in this mode)
* Only show the bell with unread notifications.
BUG=305398
Review URL: https://codereview.chromium.org/
147343003
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247318 0039d316-1c4b-4281-b951-d872f2087c98