Add SessionStateObserver with ActiveUserChanged()

ash/shelf/shelf_layout_manager_unittest.cc

Index: ash/shelf/shelf_layout_manager_unittest.cc
diff --git a/ash/shelf/shelf_layout_manager_unittest.cc b/ash/shelf/shelf_layout_manager_unittest.cc
index d6e6f281c848c1d48fea0729ad80654526bfbab7..b8ae5dbed79bcdfb75b08b092fce73d664df99f1 100644
--- a/ash/shelf/shelf_layout_manager_unittest.cc
+++ b/ash/shelf/shelf_layout_manager_unittest.cc
@@ -1246,7 +1246,7 @@ TEST_F(ShelfLayoutManagerTest, MAYBE_GestureRevealsTrayBubble) {
   generator.GestureScrollSequence(start, end,
       base::TimeDelta::FromMilliseconds(10), 1);
   EXPECT_TRUE(tray->HasSystemBubble());
-  tray->CloseSystemBubbleForTest();
+  tray->CloseSystemBubble();
   RunAllPendingInMessageLoop();
   EXPECT_FALSE(tray->HasSystemBubble());
 
@@ -1413,7 +1413,7 @@ TEST_F(ShelfLayoutManagerTest, BubbleEnlargesShelfMouseHitArea) {
     EXPECT_TRUE(shelf->IsVisible());
     if (!i) {
       // In our first iteration we make sure there is no bubble.
-      tray->CloseSystemBubbleForTest();
+      tray->CloseSystemBubble();
       EXPECT_FALSE(status_area_widget->IsMessageBubbleShown());
     } else {
       // In our second iteration we show a bubble.