Update Status Area Paint Scheduling

Update Status Area Paint Scheduling

With each TrayBackgroundView now backed by its own layer, calling SchedulePaint on StatusAreaWidgetDelegate no longer notifies them of the need to paint.

When the shelf changes its dimming state it triggers painting. Add support for StatusAreaWidget to notify each of the layer backed trays that painting is needed

TEST=Launch a non-maximized browser. The system tray will be a dark grey. Maximize the browser. The tray should be a lighter grey. This will mimic the colour changes of the application launcher on the shelf.
BUG=378115

Review URL: https://codereview.chromium.org/302153002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@274318 0039d316-1c4b-4281-b951-d872f2087c98