Don't shrink the message center height while open

ui/message_center/views/message_list_view.h

Index: ui/message_center/views/message_list_view.h
diff --git a/ui/message_center/views/message_list_view.h b/ui/message_center/views/message_list_view.h
index 1104b47cfebbfd94cf7d338000e95f9da34ae4fd..843991e8573317accf31cc9b78e8221e6c267c32 100644
--- a/ui/message_center/views/message_list_view.h
+++ b/ui/message_center/views/message_list_view.h
@@ -17,6 +17,7 @@
 #include "ui/message_center/notification.h"
 #include "ui/views/animation/bounds_animator.h"
 #include "ui/views/animation/bounds_animator_observer.h"
+#include "ui/views/controls/scroll_view.h"
 #include "ui/views/view.h"
 
 namespace ui {
@@ -55,6 +56,8 @@ class MESSAGE_CENTER_EXPORT MessageListView
   void SetRepositionTargetForTest(
       const gfx::Rect& target_rect);
 
+  void set_scroller(views::ScrollView* scroller) { scroller_ = scroller; }
+
  protected:
   // Overridden from views::View.
   void Layout() override;
@@ -77,9 +80,9 @@ class MESSAGE_CENTER_EXPORT MessageListView
   // Animates all notifications below target upwards to align with the top of
   // the last closed notification.
   void AnimateNotificationsBelowTarget();
-  // Animates all notifications above target downwards to align with the top of
-  // the last closed notification.
-  void AnimateNotificationsAboveTarget();
+  // Animates all notifications to align with the top of the last closed
+  // notification.
+  void AnimateNotifications();
   // Computes reposition offsets for |AnimateNotificationsAboveTarget|.
   std::vector<int> ComputeRepositionOffsets(const std::vector<int>& heights,
                                             const std::vector<bool>& deleting,
@@ -93,6 +96,10 @@ class MESSAGE_CENTER_EXPORT MessageListView
                     int height,
                     bool animate_even_on_move);
 
+  // Calculate the new fixed height and update with it. |requested_height|
+  // is the minimum height, and actual fixed height should be more than it.
+  void UpdateFixedHeight(int requested_height, bool prevent_scroll);
+
   // Animate clearing one notification.
   void AnimateClearingOneNotification();
 
@@ -110,6 +117,8 @@ class MESSAGE_CENTER_EXPORT MessageListView
   std::list<views::View*> clearing_all_views_;
   views::BoundsAnimator animator_;
 
+  views::ScrollView* scroller_ = nullptr;
+
   // If true, the message loop will be quitted after the animation finishes.
   // This is just for tests and has no setter.
   bool quit_message_loop_after_animation_for_test_;