[sync] Isolate FOREIGN_SESSION_UPDATED notification in ProfileSyncService

[sync] Isolate FOREIGN_SESSION_UPDATED notification in ProfileSyncService

This CL paves the way for elimination of the FOREIGN_SESSION_UPDATED
notification as part of componentizing sync code. Concretely, it replaces the
notification via a SyncServiceObserver method and changes consumers to use the
observer method instead.

Currently, ProfileSyncService triggers the observer callback by itself
listening for the FOREIGN_SESSION_UPDATED method. Once SessionsSyncManager has
been changed to communicate directly with ProfileSyncService rather than
sending the notification, the notification can be eliminated entirely.

BUG=512825
Committed: https://crrev.com/fc39f9aad7e4353304c7e2ad47f768871cd2bb83
Cr-Commit-Position: refs/heads/master@{#357061}

[blundell, 2015-10-29 14:37:51]

blundell@chromium.org changed reviewers:
+ dbeam@chromium.org, tedchoc@chromium.org, zea@chromium.org

[blundell, 2015-10-29 14:37:52]

zea: whole CL.
dbeam: //chrome/browser/ui/webui
tedchoc: //chrome/browser/android

Nicolas, I figure that this should set things up so that you can just
communicate from SessionsSyncManager to ProfileSyncService via a mechanism other
than the notification and then kill the notification entirely as part of the
componentization of SessionsSyncManager.

[Ted C, 2015-10-29 16:00:33]

On 2015/10/29 14:37:52, blundell wrote:
> zea: whole CL.
> dbeam: //chrome/browser/ui/webui
> tedchoc: //chrome/browser/android
> 
> Nicolas, I figure that this should set things up so that you can just
> communicate from SessionsSyncManager to ProfileSyncService via a mechanism
other
> than the notification and then kill the notification entirely as part of the
> componentization of SessionsSyncManager.

chrome/browser/android - lgtm

[Dan Beam, 2015-10-29 20:38:17]

lgtm

[Nicolas Zea, 2015-10-29 23:04:49]

lgtm

[Nicolas Zea, 2015-10-29 23:06:56]

LGTM. FYI I'll be removing this notification dependency once I rebase 1408353005
onto this patch.

[blundell, 2015-10-30 06:58:15]

The CQ bit was checked by blundell@chromium.org

[commit-bot: I haz the power, 2015-10-30 06:58:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1406943004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1406943004/1

[commit-bot: I haz the power, 2015-10-30 08:26:07]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-30 08:27:12]

Patchset 1 (id:??) landed as
https://crrev.com/fc39f9aad7e4353304c7e2ad47f768871cd2bb83
Cr-Commit-Position: refs/heads/master@{#357061}