Adding "UserAddedToSession" notifier to the SessionStateObserver

Adding "UserAddedToSession" notifier to the SessionStateObserver

As discussed - there is no way to figure out that a user got added to the session. The UI needs to adapt to new users by presenting them dynamically in the system shelf menu (and possibly other places).

This CL is adding this observer functionality accordingly.

BUG=305433
TEST=manual testing by adding functions to various existing observer to see if they fire upon adding user.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=227806

[Mr4D (OOO till 08-26), 2013-10-09 14:32:12]

Please have a look!

(Added Oshima as owner)

[Dmitry Polukhin, 2013-10-09 15:43:07]

lgtm

[oshima, 2013-10-09 16:39:57]

lgtm

[commit-bot: I haz the power, 2013-10-09 16:41:38]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/skuhne@chromium.org/26359003/1

[commit-bot: I haz the power, 2013-10-09 17:59:15]

Retried try job too often on android_dbg for step(s) slave_steps
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=android_db...

[commit-bot: I haz the power, 2013-10-09 19:54:38]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/skuhne@chromium.org/26359003/1

[commit-bot: I haz the power, 2013-10-09 21:01:39]

Change committed as 227806