Unified: Fix asan failure for slider bubble.

UnifiedSliderBubbleController was not handling events properly when
the previous bubble was closing e.g. volume key was pressed during
brightness slider is fading out.

TEST=manual
BUG=866301

Change-Id: I93087c01b38cbd7d5f64f8c99c9e0c539113f8d8
Reviewed-on: https://chromium-review.googlesource.com/1159924Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580088}

ash/system/unified/unified_slider_bubble_controller.cc

@@ -53,6 +53,9 @@ void UnifiedSliderBubbleController::CloseBubble() {
   slider_controller_.reset();
   if (!bubble_widget_)
     return;
+  // Ignore the request if the bubble is closing.
+  if (bubble_widget_->IsClosed())
+    return;
   bubble_widget_->Close();
   tray_->SetTrayBubbleHeight(0);
 }
@@ -104,6 +107,10 @@ void UnifiedSliderBubbleController::ShowBubble(SliderType slider_type) {
     return;
   }
 
+  // Ignore the request if the bubble is closing.
+  if (bubble_widget_ && bubble_widget_->IsClosed())
+    return;
+
   // If the bubble already exists, update the content of the bubble and extend
   // the autoclose timer.
   if (bubble_widget_) {