App list PaginationModel: Use EASE_IN_OUT tweening for smooth animation.

ui/app_list/pagination_model.cc

 // Copyright (c) 2012 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 "ui/app_list/pagination_model.h"
 
 #include <algorithm>
 
 #include "ui/app_list/pagination_model_observer.h"
 #include "ui/gfx/animation/slide_animation.h"
@@ skipping 215 matching lines @@
   return std::max(start_page, std::min(end_page, target_page));
 }
 
 void PaginationModel::StartTransitionAnimation(const Transition& transition) {
   DCHECK(selected_page_ != transition.target_page);
 
   NotifyTransitionStarted();
   SetTransition(transition);
 
   transition_animation_.reset(new gfx::SlideAnimation(this));
-  transition_animation_->SetTweenType(gfx::Tween::LINEAR);
+  transition_animation_->SetTweenType(gfx::Tween::EASE_IN_OUT);
   transition_animation_->Reset(transition_.progress);
 
   const int duration = is_valid_page(transition_.target_page) ?
       transition_duration_ms_ : overscroll_transition_duration_ms_;
   if (duration)
     transition_animation_->SetSlideDuration(duration);
 
   transition_animation_->Show();
 }
 
@@ skipping 27 matching lines @@
   } else if (transition_animation_->GetCurrentValue() == 0) {
     // Hiding animation ends. No page change should happen.
     ResetTransitionAnimation();
   }
 
   if (next_target >= 0)
     SelectPage(next_target, true);
 }
 
 }  // namespace app_list