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

ash/system/tray/hover_highlight_view.h

Index: ash/system/tray/hover_highlight_view.h
diff --git a/ash/system/tray/hover_highlight_view.h b/ash/system/tray/hover_highlight_view.h
index 4283c53564bc08ca77774cdbf58cc8bc48c0eaa2..6c60bfcdf408bf809b63330b1aa05e9b42b4e39e 100644
--- a/ash/system/tray/hover_highlight_view.h
+++ b/ash/system/tray/hover_highlight_view.h
@@ -69,8 +69,8 @@ class HoverHighlightView : public ActionableView {
   // 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