Add SystemTrayObservers

Once there is more than one SystemTray instance, we will need to support observer lists so that multiple tray items can be notified.

Hosting the observer lists in a separate class owned by ass::SystemTrayDelegate seemed like the most forward approach.

BUG=159543
For ash.gyp:
TBR=sky@chromium.org


Review URL: https://chromiumcodereview.appspot.com/11312139

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@167548 0039d316-1c4b-4281-b951-d872f2087c98
13 files changed