Refactor app list so AppsGridView owns the PaginationModel.

ui/app_list/views/page_switcher.cc

Index: ui/app_list/views/page_switcher.cc
diff --git a/ui/app_list/views/page_switcher.cc b/ui/app_list/views/page_switcher.cc
index 84077f9a6a86a46e2b588d64ce781504a059b75b..7da2593e7ca86169e9ec7af858ccb9ec2da0c059 100644
--- a/ui/app_list/views/page_switcher.cc
+++ b/ui/app_list/views/page_switcher.cc
@@ -128,7 +128,8 @@ PageSwitcher::PageSwitcher(PaginationModel* model)
 }
 
 PageSwitcher::~PageSwitcher() {
-  model_->RemoveObserver(this);
+  if (model_)
+    model_->RemoveObserver(this);
 }
 
 int PageSwitcher::GetPageForPoint(const gfx::Point& point) const {
@@ -165,6 +166,13 @@ void PageSwitcher::UpdateUIForDragPoint(const gfx::Point& point) {
   }
 }
 
+void PageSwitcher::ReleasePaginationModel() {
+  if (!model_)
+    return;
+  model_->RemoveObserver(this);
+  model_ = NULL;
+}
+
 gfx::Size PageSwitcher::GetPreferredSize() const {
   // Always return a size with correct height so that container resize is not
   // needed when more pages are added.
@@ -218,6 +226,9 @@ void PageSwitcher::CalculateButtonWidthAndSpacing(int contents_width) {
 
 void PageSwitcher::ButtonPressed(views::Button* sender,
                                  const ui::Event& event) {
+  if (!model_)
+    return;
+
   for (int i = 0; i < buttons_->child_count(); ++i) {
     if (sender == static_cast<views::Button*>(buttons_->child_at(i))) {
       model_->SelectPage(i, true /* animate */);
@@ -227,6 +238,7 @@ void PageSwitcher::ButtonPressed(views::Button* sender,
 }
 
 void PageSwitcher::TotalPagesChanged() {
+  DCHECK(model_);
   buttons_->RemoveAllChildViews(true);
   for (int i = 0; i < model_->total_pages(); ++i) {
     PageSwitcherButton* button = new PageSwitcherButton(this);
@@ -248,6 +260,7 @@ void PageSwitcher::TransitionStarted() {
 }
 
 void PageSwitcher::TransitionChanged() {
+  DCHECK(model_);
   const int current_page = model_->selected_page();
   const int target_page = model_->transition().target_page;