Shrink UnifiedSystemTray when notification removed

Different from the current notification center, UnifiedSystemTray will
shrink its area when notifications are removed.

UX spec: http://shortn/_U7QSE9nhSk

TEST=manual
BUG=828752

Change-Id: I5c793d39f88aa81fb7a2fa90366e62fc5dc9af2d
Reviewed-on: https://chromium-review.googlesource.com/1011522Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550476}

ash/message_center/message_list_view.cc

@@ -47,6 +47,7 @@ MessageListView::MessageListView()
       fixed_height_(0),
       has_deferred_task_(false),
       clear_all_started_(false),
+      use_fixed_height_(true),
       animator_(this),
       weak_ptr_factory_(this) {
   auto layout = std::make_unique<views::BoxLayout>(views::BoxLayout::kVertical,
@@ -207,7 +208,7 @@ gfx::Size MessageListView::CalculatePreferredSize() const {
 }
 
 int MessageListView::GetHeightForWidth(int width) const {
-  if (fixed_height_ > 0)
+  if (use_fixed_height_ && fixed_height_ > 0)
     return fixed_height_;
 
   width -= GetInsets().width();

ash/message_center/message_list_view.h

@@ -68,6 +68,9 @@ class ASH_EXPORT MessageListView : public views::View,
   void AddObserver(Observer* observer);
   void RemoveObserver(Observer* observer);
 
+  void set_use_fixed_height(bool use_fixed_height) {
+    use_fixed_height_ = use_fixed_height;
+  }
   void set_scroller(views::ScrollView* scroller) { scroller_ = scroller; }
 
  protected:
@@ -128,6 +131,7 @@ class ASH_EXPORT MessageListView : public views::View,
   int fixed_height_;
   bool has_deferred_task_;
   bool clear_all_started_;
+  bool use_fixed_height_;
   std::set<views::View*> adding_views_;
   std::set<views::View*> deleting_views_;
   std::set<views::View*> deleted_when_done_;

ash/system/unified/unified_message_center_view.cc

@@ -46,6 +46,7 @@ UnifiedMessageCenterView::UnifiedMessageCenterView(
   scroller_->SetHorizontalScrollBar(new views::OverlayScrollBar(true));
   AddChildView(scroller_);
 
+  message_list_view_->set_use_fixed_height(false);
   message_list_view_->set_scroller(scroller_);
   scroller_->SetContents(message_list_view_);