shelf: Hide navigation widget background when buttons are hidden

If no buttons are shown in the shelf navigation widget, we should hide
the widget background, too.

BUG=1012814

Change-Id: I6763c6719d256b2b07b74c06948af96074ae4851
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2025636Reviewed-by: Manu Cornet <manucornet@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736007}

ash/shelf/shelf_layout_manager.cc

@@ -1639,6 +1639,7 @@ void ShelfLayoutManager::CalculateTargetBounds(
 
   // Enlarge the widget to take up available space, this ensures events which
   // are outside of the HomeButton bounds can be received.
+  if (!nav_size.IsEmpty())
     nav_size.Enlarge(home_button_edge_spacing, home_button_edge_spacing);
 
   if (shelf_->IsHorizontalAlignment() && base::i18n::IsRTL())

ash/shelf/shelf_navigation_widget.cc

@@ -173,11 +173,18 @@ void ShelfNavigationWidget::Delegate::Init(ui::Layer* parent_layer) {
 void ShelfNavigationWidget::Delegate::UpdateOpaqueBackground() {
   opaque_background_.SetColor(ShelfConfig::Get()->GetShelfControlButtonColor());
 
+  // Hide background if no buttons should be shown.
+  if (!IsHomeButtonShown() && !IsBackButtonShown()) {
+    opaque_background_.SetVisible(false);
+    return;
+  }
+
   if (chromeos::switches::ShouldShowShelfHotseat() && IsTabletMode() &&
       ShelfConfig::Get()->is_in_app()) {
     opaque_background_.SetVisible(false);
     return;
   }
+
   opaque_background_.SetVisible(true);
 
   int radius = ShelfConfig::Get()->control_border_radius();