Enable mash shelf tooltips.

ash/mus/shelf_delegate_mus.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/mus/shelf_delegate_mus.h"
 
 #include "ash/shelf/shelf.h"
 #include "ash/shelf/shelf_item_delegate.h"
 #include "ash/shelf/shelf_item_delegate_manager.h"
 #include "ash/shelf/shelf_layout_manager.h"
@@ skipping 48 matching lines @@
   ShelfMenuModel* CreateApplicationMenu(int event_flags) override {
     NOTIMPLEMENTED();
     return nullptr;
   }
 
   bool IsDraggable() override {
     NOTIMPLEMENTED();
     return false;
   }
 
-  bool ShouldShowTooltip() override {
-    // NOTIMPLEMENTED(); very noisy
-    return false;
-  }
+  bool ShouldShowTooltip() override { return true; }
 
   void Close() override { NOTIMPLEMENTED(); }
 
   // TODO(msw): Support multiple open windows per button.
   uint32_t window_id_;
   base::string16 title_;
   UserWindowController* user_window_controller_;
 
   DISALLOW_COPY_AND_ASSIGN(ShelfItemDelegateMus);
 };
@@ skipping 123 matching lines @@
   DCHECK_GE(index, 0);
   ShelfItems::const_iterator iter = model_->ItemByID(shelf_id);
   DCHECK(iter != model_->items().end());
   ShelfItem item = *iter;
   item.status = has_focus ? STATUS_ACTIVE : STATUS_RUNNING;
   model_->Set(index, item);
 }
 
 }  // namespace sysui
 }  // namespace ash