App list PaginationModel: Use EASE_IN_OUT tweening for smooth animation.

BUG=406222

Review URL: https://codereview.chromium.org/557173003

Cr-Commit-Position: refs/heads/master@{#294129}

ui/app_list/pagination_model.cc

@@ -233,7 +233,7 @@ void PaginationModel::StartTransitionAnimation(const Transition& transition) {
   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) ?