Don't call AnimateClearingOneNotification if clearing all is already started.

ui/message_center/views/message_list_view.cc

 // Copyright (c) 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/command_line.h"
 #include "base/location.h"
 #include "base/single_thread_task_runner.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "ui/gfx/animation/slide_animation.h"
 #include "ui/message_center/message_center_style.h"
@@ skipping 294 matching lines @@
   gfx::Rect child_area = GetContentsBounds();
   if (child_area.IsEmpty())
     return;
 
   if (animator_.IsAnimating()) {
     has_deferred_task_ = true;
     return;
   }
 
   if (!clearing_all_views_.empty()) {
-    AnimateClearingOneNotification();
+    if (!clear_all_started_)
+      AnimateClearingOneNotification();
     return;
   }
 
   int new_height = GetHeightForWidth(child_area.width() + GetInsets().width());
   SetSize(gfx::Size(child_area.width() + GetInsets().width(), new_height));
 
   if (base::CommandLine::ForCurrentProcess()->HasSwitch(
           switches::kEnableMessageCenterAlwaysScrollUpUponNotificationRemoval))
     AnimateNotificationsBelowTarget();
   else
@@ skipping 207 matching lines @@
         base::TimeDelta::FromMilliseconds(
             kAnimateClearingNextNotificationDelayMS));
   }
 }
 
 void MessageListView::SetRepositionTargetForTest(const gfx::Rect& target_rect) {
   SetRepositionTarget(target_rect);
 }
 
 }  // namespace message_center