[Ash] Disallow calling HoverHighlightView::SetSubText() with an empty string

ash/system/tray/hover_highlight_view.h

 // Copyright 2013 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.
 
 #ifndef ASH_SYSTEM_TRAY_HOVER_HIGHLIGHT_VIEW_H_
 #define ASH_SYSTEM_TRAY_HOVER_HIGHLIGHT_VIEW_H_
 
 #include "ash/system/tray/actionable_view.h"
 #include "ash/system/tray/tray_popup_item_style.h"
 #include "base/macros.h"
@@ skipping 51 matching lines @@
   // the other Add* functions first). |icon_size| is the size of the icon in DP.
   void AddRightIcon(const gfx::ImageSkia& image, int icon_size);
 
   // Add an optional right view to an already established view (call one of
   // the other Add* functions first).
   void AddRightView(views::View* view);
 
   // Hide or show the right view.
   void SetRightViewVisible(bool visible);
 
-  // Sets text for the sub label. Precondition for this function is that
-  // |text_label_| is non-null.
+  // Sets the text of |sub_text_label_| to |sub_text|. Prior to calling this
+  // function, |text_label_| must not be null and |sub_text| must not be empty.
   void SetSubText(const base::string16& sub_text);
 
   // Allows view to expand its height. Size of unexapandable view is fixed and
   // equals to kTrayPopupItemHeight.
   void SetExpandable(bool expandable);
 
   // Changes the view's current accessibility state. This will fire an
   // accessibility event if needed.
   void SetAccessiblityState(AccessibilityState accessibility_state);
 
@@ skipping 38 matching lines @@
   TriView* tri_view_ = nullptr;
   bool expandable_ = false;
   AccessibilityState accessibility_state_ = AccessibilityState::DEFAULT;
 
   DISALLOW_COPY_AND_ASSIGN(HoverHighlightView);
 };
 
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_TRAY_HOVER_HIGHLIGHT_VIEW_H_