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

ash/system/tray/hover_highlight_view.cc

Index: ash/system/tray/hover_highlight_view.cc
diff --git a/ash/system/tray/hover_highlight_view.cc b/ash/system/tray/hover_highlight_view.cc
index e96272b6761e84dc7390fca4249f3bf68588ab77..b973129f9c89d8ac3b0b465bee891e6bb77b4ba6 100644
--- a/ash/system/tray/hover_highlight_view.cc
+++ b/ash/system/tray/hover_highlight_view.cc
@@ -56,12 +56,16 @@ void HoverHighlightView::SetRightViewVisible(bool visible) {
 }
 
 void HoverHighlightView::SetSubText(const base::string16& sub_text) {
+  if (sub_text.empty())

[mohsen, 2017/04/27 19:39:37]

We should either disallow empty string altogether (by DCHECKing) or at least
clear the |sub_text_label_|'s text if it already exists (if we don't want to
remove the label entirely). WDYT?

[tdanderson, 2017/04/27 20:52:16]

Done.

+    return;
+
   DCHECK(text_label_);
 
   if (!sub_text_label_) {
     sub_text_label_ = TrayPopupUtils::CreateDefaultLabel();
     tri_view_->AddView(TriView::Container::CENTER, sub_text_label_);
   }
+
   TrayPopupItemStyle sub_style(TrayPopupItemStyle::FontStyle::CAPTION);
   sub_style.set_color_style(TrayPopupItemStyle::ColorStyle::INACTIVE);
   sub_style.SetupLabel(sub_text_label_);