Refactor MessageCenterSettingsController.
The CL does refactoring for MessageCenterSettingsController to get rid
of #ifdef macros for CHROME_OS.
1. Define NotifierSource abstract class that provides a list of specific
type of notifiers.
2. Define subclass for each notifier type (APPLICATION, WEB_PAGE,
SYSTEM_COMPONENT, ARC_APPLICATION).
3. Let MessageCenterSettingsController delegate to each source.
BUG=
620184
TEST=None
Committed:
https://crrev.com/d36c21d3e9698566fac911dde6473d1023009caa
Cr-Commit-Position: refs/heads/master@{#400867}