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

SetSubText() in HoverHighlight view should not be called
if the parameter string is empty, otherwise multi-line
rows in the system menu will have the incorrect layout
of their primary text.

BUG=715615
TEST=manual

Review-Url: https://codereview.chromium.org/2844933003
Cr-Commit-Position: refs/heads/master@{#467813}

ash/system/tray/hover_highlight_view.cc

@@ -57,11 +57,13 @@ void HoverHighlightView::SetRightViewVisible(bool visible) {
 
 void HoverHighlightView::SetSubText(const base::string16& sub_text) {
   DCHECK(text_label_);
+  DCHECK(!sub_text.empty());
 
   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_);
@@ -117,7 +119,8 @@ void HoverHighlightView::DoAddIconAndLabels(
   style.SetupLabel(text_label_);
   tri_view_->AddView(TriView::Container::CENTER, text_label_);
 
-  SetSubText(sub_text);
+  if (!sub_text.empty())
+    SetSubText(sub_text);
 
   tri_view_->SetContainerVisible(TriView::Container::END, false);
 

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