Foreign tab suggestions should update when disabling sync or signing out.

components/ntp_snippets/sessions/tab_delegate_sync_adapter.cc

Index: components/ntp_snippets/sessions/tab_delegate_sync_adapter.cc
diff --git a/components/ntp_snippets/sessions/tab_delegate_sync_adapter.cc b/components/ntp_snippets/sessions/tab_delegate_sync_adapter.cc
index 4ddd3920f269cfa6ca37f057b01cdfa29f2aee11..063c113ca517af8b0100acc293b0d25139ca98ae 100644
--- a/components/ntp_snippets/sessions/tab_delegate_sync_adapter.cc
+++ b/components/ntp_snippets/sessions/tab_delegate_sync_adapter.cc
@@ -45,18 +45,40 @@ void TabDelegateSyncAdapter::SubscribeForForeignTabChange(
 }
 
 void TabDelegateSyncAdapter::OnStateChanged() {
-  // Ignored.
+  // OnStateChanged gets called very frequently, and usually is not important.
+  // But there are some events, like disabling sync and signing out, that are
+  // only captured through OnStateChange. In an attempt to send as few messages
+  // as possible, track if there was session data, and always/only invoke the
+  // calback when transitioning between states. This will also capture the case
+  // when Open Tab is added/removed from syncing types. Note that this requires
+  // the object behind GetOpenTabsUIDelegate() to have its real data when it
+  // becomes availabile. Otherwise we might transition to think we have session
+  // data, but invoke our callback while the GetOpenTabsUIDelegate() returns bad
+  // results. Fourtunately,this isn'ta problem. GetOpenTabsUIDelegate() is

[maxbogue, 2016/12/19 20:35:16]

"Fourtunately,this isn'ta" -> "Fortunately, this isn't a"

+  // guarded by verifying the data type is RUNNING, which always means the
+  // sessions merge has already happened.
+  if (had_session_data_ != HasSessionsData()) {
+    InvokeCallback();
+  }
 }
 
 void TabDelegateSyncAdapter::OnSyncConfigurationCompleted() {
-  InvokeCallback();
+  // Ignored. This event can let us know when the set of enabled data types
+  // change. However, we want to avoid useless notifications as much as
+  // possible, and all of the information captured in this event will also be
+  // covered by OnStateChange.
 }
 
 void TabDelegateSyncAdapter::OnForeignSessionUpdated() {
+  // Foreign tab data changed, always invoke the callback to generate new
+  // suggestions. Interestingly, this is only triggered after sync model type
+  // apply, not after merge. The merge case should always be handled by
+  // OnStateChange.
   InvokeCallback();
 }
 
 void TabDelegateSyncAdapter::InvokeCallback() {
+  had_session_data_ = HasSessionsData();
   if (!change_callback_.is_null()) {
     change_callback_.Run();
   }